Fully replace a scale if it is of a different type than the existing

If a plot has a continuous scale and a discrete is added, the continuous
scale is completely removed, even if it was merged before.

E.g.

ggplot() + scale_y_continuous() + scale_y_discrete()

scale_y_continuous would be fully replaced
Fully replace a scale if it is of a different type than the existing

If a plot has a continuous scale and a discrete is added, the continuous
scale is completely removed, even if it was merged before.

E.g.

ggplot() + scale_y_continuous() + scale_y_discrete()

scale_y_continuous would be fully replaced