What words should we normally use for x-axis in a economic or finance research paper?

Question

What words should we normally use for x-axis in a economic or finance research paper? For example, in this case (I draw the graph from my own reseach)

In my case, I use:

That blue dots mainly lie under the x-axis implies the decrease in y variable after the event date where the parallel trend assumption is satisfied.

I am wondering if there is any word commonly-used in economic or finance research paper standing for "x-axis" or "x-axis" is already okay to be used?

When saying "x-axis", I mean y=0

Answer 1

I find "horizontal axis" better than x-axis, because it does not use the name of a potential variable.

However, I think your whole sentence focuses to much on what we see, and not on the actual phenomenon/statistics. I will try to get the impression across by exaggerating it:
"Once the squiggly line turns blue, it usually stays below the horizontal line."
This is true, but perhaps you will agree that it does not sound very informative.

You could write that R% of the observations preceeding the event have larger than 0 causal effect while B% of the observations succeeding the event have larger than 0 causal effect. This way the reader can tell whether the change in the below zero ratio is large.

However, the above still does not say anything about statistical significance. I would guess that depending on the exact methodology/hypothesis one could find better statistics than just these ratios.