Age Bias Plots in ggplot2

Flexibly construct age bias and age difference plots using ggplot2
Age
Age Comparisons
ggplot2
Author

Michael Lant

Published

Mar 15, 2021

Modified

Dec 7, 2022

Note

The following packages are loaded for use below. I also set the default ggplot theme to theme_bw() for a classic “black-and-white” plot (rather than the default plot with a gray background).

library(FSA)          # for WhitefishLC, ageBias(), plotAB()
library(dplyr)        # for mutate()
library(ggplot2)
theme_set(theme_bw())

 

Introduction

Age-bias and difference plots can be constructed with plot() and plotAB() in the FSA package. However, these types of plots can be more flexibly constructed using ggplot2. Below I will use ggplot2 to recreate many of the plots shown in the documentation for plot() and plotAB().

 

Data

The WhitefishLC data frame from FSA.1 contains age readings made by two readers on scales, fin rays, and otoliths, along with consensus readings for each structure.

  • 1 These data are loaded automatically with library(FSA).

    • head(WhitefishLC)
    #R|    fishID  tl scale1 scale2 scaleC finray1 finray2 finrayC otolith1 otolith2
#R|  1      1 345      3      3      3       3       3       3        3        3
#R|  2      2 334      4      3      4       3       3       3        3        3
#R|  3      3 348      7      5      6       3       3       3        3        3
#R|  4      4 300      4      3      4       3       2       3        3        3
#R|  5      5 330      3      3      3       4       3       4        3        3
#R|  6      6 316      4      4      4       2       3       3        6        5
#R|    otolithC
#R|  1        3
#R|  2        3
#R|  3        3
#R|  4        3
#R|  5        3
#R|  6        6

    Intermediate and summary statistics for the comparison of paired ages (e.g., between consensus scale and otolith ages) can be extracted from the objected returned by ageBias() from FSA.2

  • 2 As described in the documentation.

    • ab1 <- ageBias(scaleC~otolithC,data=WhitefishLC,
               ref.lab="Otolith Age",nref.lab="Scale Age")

    For example, the $data object of ab13 contains the original paired age estimates, the differences between those two estimates, and the mean of those two estimates.

  • 3 ab1 because that was the name assigned to the results from ageBias() in this example.

    • head(ab1$data)
    #R|    scaleC otolithC diff mean
#R|  1      3        3    0  3.0
#R|  2      4        3    1  3.5
#R|  3      6        3    3  4.5
#R|  4      4        3    1  3.5
#R|  5      3        3    0  3.0
#R|  6      4        6   -2  5.0

    In addition, the $bias object of ab1 contains summary statistics of ages for the first structure given in the ageBias() formula by each age of the second structure given in that formula. For example, the first row below gives the number, minimum, maximum, mean, and standard error of the scale ages that were paired with an otolith age of 1. Additionally there is a t-test, adjusted p-value, and a significance statement for testing whether the mean scale age is different from the otolith age. Finally, confidence intervals (defaults to 95%) for the mean scale age at an otolith age of 1 is given, with a statement about whether a confidence interval could be calculated.4

  • 4 See the documentation for ageBias() for the criterion used to decide if the confidence interval can be calculated.

    • head(ab1$bias)
    #R|    otolithC  n min max     mean        SE          t   adj.p   sig       LCI
#R|  1        1  9   1   2 1.444444 0.1756821  2.5298218 0.28212 FALSE 1.0393208
#R|  2        2  7   1   5 2.000000 0.5773503  0.0000000 1.00000 FALSE 0.5872748
#R|  3        3 17   1   6 3.352941 0.2416423  1.4605937 0.81743 FALSE 2.8406824
#R|  4        4 18   2   6 3.833333 0.2322102 -0.7177407 1.00000 FALSE 3.3434126
#R|  5        5  8   4   8 5.250000 0.4909902  0.5091751 1.00000 FALSE 4.0889926
#R|  6        6 10   3   6 4.600000 0.2666667 -5.2500003 0.00686  TRUE 3.9967581
#R|         UCI canCI
#R|  1 1.849568  TRUE
#R|  2 3.412725  TRUE
#R|  3 3.865200  TRUE
#R|  4 4.323254  TRUE
#R|  5 6.411007  TRUE
#R|  6 5.203242  TRUE

    The results in $bias.diff are similar to those for $bias except that the difference in age between the two structures is summarized for each otolith age.

    head(ab1$bias.diff)
    #R|    otolithC  n min max       mean        SE          t   adj.p   sig         LCI
#R|  1        1  9   0   1  0.4444444 0.1756821  2.5298218 0.28212 FALSE  0.03932075
#R|  2        2  7  -1   3  0.0000000 0.5773503  0.0000000 1.00000 FALSE -1.41272519
#R|  3        3 17  -2   3  0.3529412 0.2416423  1.4605937 0.81743 FALSE -0.15931758
#R|  4        4 18  -2   2 -0.1666667 0.2322102 -0.7177407 1.00000 FALSE -0.65658738
#R|  5        5  8  -1   3  0.2500000 0.4909902  0.5091751 1.00000 FALSE -0.91100742
#R|  6        6 10  -3   0 -1.4000000 0.2666667 -5.2500003 0.00686  TRUE -2.00324188
#R|           UCI canCI
#R|  1  0.8495680  TRUE
#R|  2  1.4127252  TRUE
#R|  3  0.8652000  TRUE
#R|  4  0.3232540  TRUE
#R|  5  1.4110074  TRUE
#R|  6 -0.7967581  TRUE

    These data frames are used in ggplot2 code below to create various versions of age-bias and difference plots.

    Important

    At times multiple data frames will be used when constructing the same plot so that layers of the plot can have different variables.

     

    Basic Age-Bias Plot

    Figure 1 is the age-bias plot created by default by plotAB() from FSA.

    FSA::plotAB(ab1)

    Default age-bias plot from plotAB() in FSA.

    Figure 1: Default age-bias plot from plotAB() in FSA.

     

    Figure 1 is largely recreated (Figure 2) with the following ggplot2 code.

    ggplot(data=ab1$bias) +
  geom_abline(slope=1,intercept=0,linetype="dashed",color="gray") +
  geom_errorbar(aes(x=otolithC,ymin=LCI,ymax=UCI,color=sig),width=0) +
  geom_point(aes(x=otolithC,y=mean,color=sig,fill=sig),shape=21) +
  scale_fill_manual(values=c("FALSE"="black","TRUE"="white"),guide="none") +
  scale_color_manual(values=c("FALSE"="black","TRUE"="red3"),guide="none") +
  scale_x_continuous(name=ab1$ref.lab,breaks=0:25) +
  scale_y_continuous(name=ab1$nref.lab,breaks=0:25)

    Recreation of the default age-bias plot using ggplot2.

    Figure 2: Recreation of the default age-bias plot using ggplot2.

    The specifics of the code above are described below.

    • The base data used in this plot is the $bias data.frame discussed above.
    • The 45o agreement line (i.e., slope of 1 and intercept of 0) is added with geom_abline(), using a dashed linetype= and a gray color=. This “layer” is first so that it sits behind the other results.
    • Error bars are added with geom_errorbar(). The aes()thetics here map the consensus otolith age to the x= axis and the lower and upper confidence interval values for the mean consensus scale age at each consensus otolith age to ymin= and ymax=. The color= of the lines are mapped to the sig variable so that points that are significantly different from the 45o agreement line will have a different color (with scale_color_manual() described below). Finally, width=0 assures that the error bars will not have “end caps.”
    • Points at the mean consensus scale age (y=) for each otolith age (x=) are added with geom_point(). Again, the color= and fill= are mapped to the sig variable so that they will appear different depending on whether the points are significantly different from the 45o agreement line or not. Finally, shape=21 represents a plotted point as an open circle that is outlined with color= and filled with fill=.
    • scale_fill_manual() and scale_color_manual() are used to set the colors and fills for the levels in the sig variable. Note that guide="none" is used so that a legend is not constructed for the colors and fills.
    • scale_x_continuous() and scale_y_continuous() are used to set the labels (with name=) and axis breaks for the x- and y-axes, respectively. The names are drawn from labels that were given in the original call to ageBias() and stored in ab1.

    The gridlines and the size of the fonts could be adjusted by modifying theme theme, which I did not do here for simplicity.

     

    More Examples

    Below are more examples of how ggplot2 can be used to recreate graphs from plot() in FSA. For example, Figure 3 is similar to Figure 2, but uses $bias.diff from ab1 to plot mean differences between scale and otolith ages against otolith ages. The reference for differences is a horizontal line at 0 so geom_abline() from above was replaced with geom_hline() here.

    ggplot(data=ab1$bias.diff) +
  geom_hline(yintercept=0,linetype="dashed",color="gray") +
  geom_errorbar(aes(x=otolithC,ymin=LCI,ymax=UCI,color=sig),width=0) +
  geom_point(aes(x=otolithC,y=mean,color=sig,fill=sig),shape=21) +
  scale_fill_manual(values=c("FALSE"="black","TRUE"="white"),guide="none") +
  scale_color_manual(values=c("FALSE"="black","TRUE"="red3"),guide="none") +
  scale_x_continuous(name=ab1$ref.lab,breaks=0:25) +
  scale_y_continuous(name=paste(ab1$nref.lab,"-",ab1$ref.lab),breaks=-15:5)

    Age difference plot using ggplot2.

    Figure 3: Age difference plot using ggplot2.

     

    Figure 4 is similar but it includes the raw data points from $data and colors the mean (and confidence intervals) for the differences based on the significance as in Figure 2. Because data were drawn from different data frames (i.e., ab1$data and ab1$bias.diff) the data= and aes= arguments had to be moved into the specific geom_s. Also note that the raw data were made semi-transparent (with alpha=0.1) to emphasize the over-plotting of the discrete ages.

    ggplot() +
  geom_hline(yintercept=0,linetype="dashed",color="gray") +
  geom_point(data=ab1$data,aes(x=otolithC,y=diff),alpha=0.1,size=1.75) +
  geom_errorbar(data=ab1$bias.diff,aes(x=otolithC,ymin=LCI,ymax=UCI,color=sig),
                width=0) +
  geom_point(data=ab1$bias.diff,aes(x=otolithC,y=mean,color=sig,fill=sig),
             shape=21,size=1.75) +
  scale_fill_manual(values=c("FALSE"="black","TRUE"="white"),guide="none") +
  scale_color_manual(values=c("FALSE"="black","TRUE"="red3"),guide="none") +
  scale_x_continuous(name=ab1$ref.lab,breaks=seq(0,25,1)) +
  scale_y_continuous(name=paste(ab1$nref.lab,"-",ab1$ref.lab),breaks=-15:5)

    Age difference plot using `ggplot2` including points for individual observations.

    Figure 4: Age difference plot using ggplot2 including points for individual observations.

     

    Figure 5 is the same as Figure 4 except that a loess smoother has been added with geom_smooth() to emphasize the trend in the differences in ages. The smoother should be fit to the raw data so be sure to use ab1$data in geom_smooth(). The smoother defaults to blue (as shown here) but I decreased the width of the line slightly with linewidth=0.65.

    ggplot() +
  geom_hline(yintercept=0,linetype="dashed",color="gray") +
  geom_point(data=ab1$data,aes(x=otolithC,y=diff),alpha=0.1,size=1.75) +
  geom_errorbar(data=ab1$bias.diff,aes(x=otolithC,ymin=LCI,ymax=UCI,color=sig),
                width=0) +
  geom_point(data=ab1$bias.diff,aes(x=otolithC,y=mean,color=sig,fill=sig),
             shape=21,size=1.75) +
  scale_fill_manual(values=c("FALSE"="black","TRUE"="white"),guide="none") +
  scale_color_manual(values=c("FALSE"="black","TRUE"="red3"),guide="none") +
  scale_x_continuous(name=ab1$ref.lab,breaks=seq(0,25,1)) +
  scale_y_continuous(name=paste(ab1$nref.lab,"-",ab1$ref.lab),breaks=-15:5) +
  geom_smooth(data=ab1$data,aes(x=otolithC,y=diff),linewidth=0.65)

    Age difference plot using ggplot2 including points for individual observations and a loess smoother.

    Figure 5: Age difference plot using ggplot2 including points for individual observations and a loess smoother.

     

    What Prompted This

    Graphics made in ggplot2 are more flexible than the ones produced in FSA. For example, a user recently asked if it was possible to make an “age-bias plot” that used “error bars” based on the standard deviation rather than the standard error. While it is questionable whether this is what should be plotted, it is nevertheless up to the user and their use case. Because this cannot be done using the plots in FSA we turned to ggplot to make such a graph.

    Standard deviations are not returned in any of the ageBias() results (saved in ab1). However, the standard error and sample size are returned in the $bias data frame. The standard deviation can be “back-calculated” from these two values using SD=SE*sqrt(n). Two new variables called LSD and USD that are the means minus and plus two standard deviations can then be created. All three of these variables are added to the $bias data frame using mutate() from dplyr.

    ab1$bias <- ab1$bias |>
  mutate(SD=SE*sqrt(n),
         LSD=mean-2*SD,
         USD=mean+2*SD)

    A plot (Figure 6) like the very first plot above but using two standard deviations for the error bars is then created by mapping ymin= and ymax= to LSD and USD, respectively, in geom_errorbar(). Note that I removed the color related to the significance test as those don’t pertain to the results when using standard deviations to represent “error bars.”

    ggplot(data = ab1$bias)+
  geom_abline(slope=1,intercept=0,linetype="dashed",color="gray") +
  geom_errorbar(aes(x=otolithC,ymin=LSD,ymax=USD),width=0) +
  geom_point(aes(x=otolithC,y=mean)) +
  scale_x_continuous(name =ab1$ref.lab,breaks=0:25) +
  scale_y_continuous(name=ab1$nref.lab,breaks=0:25)

    Mean scale age for each otolith age with error bars represented by two standard deviations.

    Figure 6: Mean scale age for each otolith age with error bars represented by two standard deviations.

    Finally, to demonstrate the flexibility of using ggplot with these type of data, I used a violin plot to show the distribution of scale ages for each otolith age while also highlighting the mean scale age for each otolith age (Figure 7). The violin plots are created with geom_violin() using the raw data stored in $data. The group= must be set to the x-axis variable (i.e., otolith age) so that a separate violin will be constructed for each age on the x-axis. I filled the violins with grey to make them stand out more.

    ggplot() +
  geom_abline(slope=1,intercept=0,linetype="dashed",color="gray") +
  geom_violin(data=WhitefishLC,aes(x=otolithC,y=scaleC,group=otolithC),
              fill="grey") +
  geom_point(data=ab1$bias,aes(x=otolithC,y=mean),size=2) +
  scale_x_continuous(name=ab1$ref.lab,breaks=0:25) +
  scale_y_continuous(name=ab1$nref.lab,breaks=0:25)

    Violin plots and mean scale age at each otolith age.

    Figure 7: Violin plots and mean scale age at each otolith age.

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    Citation

    BibTeX citation:
    @online{lant2021,
  author = {Lant, Michael},
  title = {Age {Bias} {Plots} in Ggplot2},
  date = {2021-03-15},
  url = {https://fishr-core-team.github.io/fishR//blog/posts/2021-3-15-AgeBiasPlots},
  langid = {en}
}
    For attribution, please cite this work as:
    Lant, M. 2021, March 15. Age Bias Plots in ggplot2. https://fishr-core-team.github.io/fishR//blog/posts/2021-3-15-AgeBiasPlots.