I think there's no way to show only one dimension on the chart.. since the setting for the x-axis is for both the dimension. I think the only way you can do as workaround is to not show the title and add a small text&image object below the chart with the word 'Date'.. although it will eat a bit of space.
This is unfortunately not possible.
As Yddona mentioned, try to not show any label in the x-axis since everyone can see you're working with Date.
Thus, there is no need to mention it.
For the coloring part, you can explain it in the subtitles.
It's a kind of a workaround but hope this helps.