[query/ggplot] fixes math for continuous color scales

[iris-garden]

CHANGELOG: Fixed a bug in hail.ggplot.scale_color_continuous that sometimes caused errors by generating invalid colors.

For example, this code:

import hail as hl
from hail.ggplot import ggplot, aes, geom_point
t = hl.utils.range_table(10)
t = t.annotate(x=hl.rand_unif(), y=hl.rand_unif(), z=hl.rand_unif(1, 10))
fig = ggplot(t, aes(x=t.x, y=t.y, color=t.z)) + geom_point()
fig.show()

Produces an error like this:

ValueError:
    Invalid element(s) received for the 'color' property of scatter.marker
        Invalid elements include: ['rgb(-84, -187, 123)', 'rgb(-116, -227, 131)', 'rgb(-29, -120, 109)', 'rgb(-33, -125, 110)', 'rgb(15, -65, 97)', 'rgb(10, -71, 99)', 'rgb(-112, -223, 130)', 'rgb(-63, -162, 117)', 'rgb(-81, -185, 122)', 'rgb(31, -45, 93)']

But after this change is applied, it produces a plot like this: