Inconsistent y-axis labeling for diagonal density plots in ggpairs()

[Liu499-create]

Issue Description:
When using ggpairs() with density plots on the diagonal, only the first variable's density plot shows proper density scale on the y-axis, while other variables display their raw value ranges instead of density values. This appears inconsistent with the expected behavior.

Reproducible Example:

library(GGally)
library(ggplot2)

data("mtcars", package = "datasets")
mtcars_subset = subset(mtcars, select = c("mpg", "cyl", "disp"))

ggpairs(
mtcars_subset,
axisLabels = "show",
diag = list(continuous = wrap("densityDiag"))
)

Observed Behavior:

1. For mpg (first variable):

• Left y-axis shows density values (0.00 to 0.06)
• Bottom x-axis shows mpg values (10 to 35)

2. For cyl and disp:

• Left y-axis shows variable ranges (4-8 for cyl, 100-400 for disp)
• No visible density scale
• Bottom x-axis shows respective variable values

Expected Behavior:
All diagonal density plots should:

• Show density values on the y-axis (like mpg does)
• Show variable values on the x-axis
• Consistently display both axes for all variables