is_discrete: a rounding error.

[alshan]

See geom_smooth.ipynb

A rounding error occurs when using 'is_discrete'. However, its not clear whether this is caused by 'is_discrete' or not.

[alshan]

I've patched DataProcessing (https://github.com/JetBrains/lets-plot/commit/e7c5ca7e5f7a4211fe8c9cf6227b2f2ecfbde33f).

Needs further investigation:

• Why data.isNumeric(variable) returns True. Expected False for as_discrete(var)
• It seems that two different color scales are created for points layer and for lines layer. Expected - one color scale per plot.

[IKupriyanov-HORIS]

Fixed rounding error, but color scales issue is not related with as_discrete function and requires separate issue: