Statistical Test Plotting with ‘ggasym’

Joshua H. Cook

2020-05-31

Purpose

One of the great uses of ‘ggasym’ is to plot two values from the results of a multi-way statistical test. Each comparison is a cell, and two values can be used for the fills. Below I give brief examples and plot the differences in mean and the p-value on the symmetric matrix.

library(dplyr)
library(ggplot2)
library(tibble)
library(purrr)
library(broom)
library(ggasym)

Data

The data will be modeled as expression values of 6 genes, each with 10 measurements. I will then as if any of them have different levels of expression.

n_reps <- 10  # number of measurements per gene
expt_std_dev <- 1.5  # std. dev. of measurements
genes <- c("FAK", "talin", "paxillin", "vinculin", "B1integrin", "kindlin")
# "real" expression levels to be used as the mean in `rnorm`
real_expression_levels <- sample(seq(1, 5, 0.1), length(genes), replace = TRUE)
# create a tibble
expr_data <- tibble(gene = rep(genes, n_reps),
                    real_expr = rep(real_expression_levels, n_reps),
                    rep_num = sort(rep(1:n_reps, length(genes))))
# add in the measured expression values as a normal distribution around the mean
expr_data <- expr_data %>%
    mutate(expt_expr = rnorm(nrow(expr_data),
                             mean = real_expr,
                             sd = expt_std_dev))
head(expr_data)
#> # A tibble: 6 x 4
#>   gene       real_expr rep_num expt_expr
#>   <chr>          <dbl>   <int>     <dbl>
#> 1 FAK              2.3       1     2.92 
#> 2 talin            1.3       1    -1.01 
#> 3 paxillin         4.8       1     3.41 
#> 4 vinculin         1         1     0.558
#> 5 B1integrin       4.3       1     4.29 
#> 6 kindlin          3.2       1     6.81

Statistical Tests

I then use an ANOVA to test if there are any differences between any of the comparisons of gene expression.

res_aov <- aov(expt_expr ~ gene, data = expr_data)
broom::tidy(res_aov)
#> # A tibble: 2 x 6
#>   term         df sumsq meansq statistic  p.value
#>   <chr>     <dbl> <dbl>  <dbl>     <dbl>    <dbl>
#> 1 gene          5  144.  28.8       14.0  8.99e-9
#> 2 Residuals    54  111.   2.06      NA   NA

With a very low p-value from the ANOVA, I run the Tukey post-hoc test to find which genes are at different levels.

tukey_res <- TukeyHSD(res_aov)
tukey_res
#>   Tukey multiple comparisons of means
#>     95% family-wise confidence level
#> 
#> Fit: aov(formula = expt_expr ~ gene, data = expr_data)
#> 
#> $gene
#>                           diff        lwr         upr     p adj
#> FAK-B1integrin      -1.8081714 -3.7040504  0.08770770 0.0697840
#> kindlin-B1integrin  -1.2677974 -3.1636765  0.62808162 0.3694231
#> paxillin-B1integrin  0.4404870 -1.4553920  2.33636610 0.9827300
#> talin-B1integrin    -3.2562596 -5.1521387 -1.36038054 0.0000698
#> vinculin-B1integrin -3.7874196 -5.6832986 -1.89154052 0.0000036
#> kindlin-FAK          0.5403739 -1.3555051  2.43625298 0.9581279
#> paxillin-FAK         2.2486584  0.3527793  4.14453747 0.0113651
#> talin-FAK           -1.4480882 -3.3439673  0.44779083 0.2298466
#> vinculin-FAK        -1.9792482 -3.8751273 -0.08336916 0.0359225
#> paxillin-kindlin     1.7082845 -0.1875946  3.60416355 0.1000603
#> talin-kindlin       -1.9884622 -3.8843412 -0.09258309 0.0346062
#> vinculin-kindlin    -2.5196221 -4.4155012 -0.62374308 0.0032062
#> talin-paxillin      -3.6967466 -5.5926257 -1.80086758 0.0000060
#> vinculin-paxillin   -4.2279066 -6.1237857 -2.33202757 0.0000003
#> vinculin-talin      -0.5311600 -2.4270390  1.36471907 0.9610525

Plotting

Now I want to plot the estimate in the top-left and p adj in the bottom right. First, I must prepare the data for use with geom_asymmat() by passing the results of the Tukey post-hoc test to asymmetrise_stats(). You can see that it returns the data in a tibble with new columns x and y that are the result of splitting comparison.

asymmat_tib <- asymmetrise_stats(tukey_res)
head(asymmat_tib)
#> # A tibble: 6 x 9
#>   term  contrast  null.value estimate conf.low conf.high adj.p.value x     y    
#>   <chr> <chr>          <dbl>    <dbl>    <dbl>     <dbl>       <dbl> <chr> <chr>
#> 1 gene  FAK-B1in…          0   -1.81     -3.70    0.0877  0.0698     FAK   B1in…
#> 2 gene  kindlin-…          0   -1.27     -3.16    0.628   0.369      kind… B1in…
#> 3 gene  paxillin…          0    0.440    -1.46    2.34    0.983      paxi… B1in…
#> 4 gene  talin-B1…          0   -3.26     -5.15   -1.36    0.0000698  talin B1in…
#> 5 gene  vinculin…          0   -3.79     -5.68   -1.89    0.00000356 vinc… B1in…
#> 6 gene  kindlin-…          0    0.540    -1.36    2.44    0.958      kind… FAK

Finally, I can plot the data using geom_asymmat().

ggplot(asymmat_tib, aes(x = x, y = y)) +
    geom_asymmat(aes(fill_tl = estimate, fill_br = -log(adj.p.value))) +
    scale_fill_tl_gradient2(low = "dodgerblue", high = "tomato") +
    scale_fill_br_distiller(type = "seq", palette = "Greens", direction = 1)

And add a few styling changes with normal ‘ggplot2’ semantics.

ggplot(asymmat_tib, aes(x = x, y = y)) +
    geom_asymmat(aes(fill_tl = estimate, fill_br = -log(adj.p.value))) +
    scale_fill_tl_gradient2(low = "dodgerblue", high = "tomato",
                            guide = guide_colourbar(order = 1)) +
    scale_fill_br_distiller(type = "seq", palette = "Greens", direction = 1,
                            guide = guide_colourbar(order = 2)) +
    theme_bw() +
    theme(panel.background = element_rect(fill = "grey50"),
          panel.grid = element_blank(),
          axis.title = element_blank(),
          plot.title = element_text(hjust = 0.5)) +
    labs(title = "Differential Gene Expression",
         fill_tl = "diff. in\nmean expr.",
         fill_br = "-log( adj. p-value )") +
    scale_x_discrete(expand = c(0,0)) +
    scale_y_discrete(expand = c(0,0))

One of the conclusions that can be drawn here is that the difference in expression of kindlin and FAK is the greatest and has a very low adjusted p-value. Thus, one of the conclusion is that kindlin is expressed at a lower level than FAK.


For more information, see the complete documentation at the ‘ggasym’ site.