Hatching using ggplot

[richardli]

The current hatchPlot function is not very flexible (e.g., difficult to transform scales and color gradients). It would be great to implement it in ggplot. I'm keeping this thread open in case anyone wants (and has the time and curiosity!) to help. I'll post related stuff here too.

For an illustration: https://imaddowzimet.github.io/crosshatch/, which implemented hatching, but seems not easily adaptable to change hatching density and show on legends.