Arrows to visualize Axis Direction

[ImmanuelHaffner]

Hi,

I have the following layered chart, which is composed of two charts:

One of the charts produces the top x-Axis going from left to right, the other chart produces the bottom x-Axis going from right to left. To make it easier for the viewer to understand the axis, I would like to add an arrow at the end of the axis to indicate the direction.

In Ascii:

+-------->
|        |
|        |
|        |
<--------+

How can I do this with Altair?

Thanks,
Immanuel