The VTKPlotter
object returned by plot
has a zoom
method which you can use to zoom into the image, which might help reduce the amount of white space you have:
plotter = plot(u, window_height=500, window_width=700)
plotter.zoom(1.5) # zoom into the image a bit to reduce white space
plotter.write_png("image") # save image
This might require a bit of trial-and-error to figure out the right parameters though.
Alternatively, depending on your image, it might be more convenient to crop out the white space using image editing software (there are automated utilities such as ImageMagick which can do batch trimming of white space).