Keyboard shortcuts for separating the zoom-in/out of x and y axes in a chart

Currently, I am working on a plotting library to plot a few charts.
Most of the charts data vary in the x-dimension much more than in the y-dimension.
Example:

time x y
0    0  0
1    1  0.0044
2    2  0.0085

Currently, we use Ctrl + Wheel to zoom-in/out. It's good if the values on both axes vary with the same degree, but this is not the case.

I have been thinking about separating the zooming behavior, examples for schemes:

scheme 1

Ctrl + Wheel: both axes.
Ctrl + Shift + Wheel: x-axis.
Ctrl + Alt + Wheel: y-axis.

scheme 2

Mouse middle button: Switch between axis.
Ctrl + Wheel: zoom the current selected axis or both.

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I have been thinking about the schemes, but I don't think it's too intuitive.

I have to depend on being intuitive for the user interface, as the program itself is very complex with a huge user guide. And I don't want to add non-necessary documentation for simple navigation.

So are there more intuitive shortcuts for special modes in zooming?