Compound units fail

[teunbrand]

The following fails

library(ggplot2)
library(elementalist)

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme(axis.ticks = element_line_wiggle(1))
#> Error in xy[-nxy, , drop = FALSE] - xy[-1, , drop = FALSE]: non-numeric argument to binary operator

Most likely because I've been ignoring the existence of compound units, such as arithmetic units (e.g. unit(1, "cm") + unit(0.2, "npc")). I've checked this through the debug system. I don't know what the optimal solution is, but here are some thoughts:

• Delay evaluating line manipulations until draw time. This allows me to convert to absolute units first and then apply manipulations. Downside is that there is no consistency in line manipulations anymore as the plot window is resized and it makes resizing slower.

• If this is an edge case for the axis guide ticks, we could probably get away with solving this in the particular case of axis guides instead of solving the general case. The only other 'ticks'-case I can think of are the guide_colourbar() ticks, but these don't appear to be under the control of the theme system.

• Get advice from someone smarter than me on how this can be solved best.

• Error more gracefully

[teunbrand]

For now this is lazily fixed by having using guide_axis2() instead.