changing plot properties doesn't trigger the build

[fmichonneau]

I had a target with:

figures/plot1.pdf:
  command: make_figure_1()
  plot: true

that I updated to

figures/plot1.pdf:
  command: make_figure_1()
  plot:
    width: 12
    height: 8

but that didn't trigger the re-building of the figure.