Allow single-axis zoom via scroll wheel

In many cases like time series visualization it is not useful to keep the X/Y aspect ratio fixed. Instead, I usually want to zoom independently in horizontal or vertical direction. Currently, zooming via Ctrl+Mouse wheel will always affect both axis equally.

Instead, I propose to enhance the API to allow zooming each axis independently from each other, something like
```
/// Default is off (current behavior)
pub fn enable_zoom_per_axis(self, on: bool) -> Self
```
If enabled, Ctrl-Wheel would zoom only either axis (not sure which one should be picked), Ctrl-Shift-Wheel only the other one.

There is a related request #17 which would allow per-axis zooming by dragging the axis. I don't consider it strictly as an alternative, but complementary, hence I would favor if both proposals would be realized.