`scale_color_gradient`: Tooltip formatting does not inherit from scale formatting

[ASmirnov-HORIS]

Example:

data = {
    'x': [0, 1],
    'y': [0, 1],
    'g': [0, 1e10],
}
ggplot(data, aes('x', 'y', color='g')) + geom_point() + scale_color_gradient(format=".2~s")

Output:

Expected: