Vline! alters y-axis labels using Plots with GR

Hi all,

An MWE follows:

using Plots
dt = [ today() + Day(10*n) for n = 1:100 ];
x = randn(100);
p = plot(dt, x);
savefig(p, "/home/colin/TempPlot/a1.svg");
dtlab = dt[1:20:100];
vline!(p, dtlab);
savefig(p, "/home/colin/TempPlot/a2.svg");

In the second plot, the vertical lines are inserted in the correct location, but for some reason the y-axis labels have changed from floating point numbers to dates…

Is this expected behaviour? Am I overlooking something obvious here?