How to adjust axis ticks interval in heat map? and How to draw a line in heat map?

I have a heat map of Roi(y) traces over time(x). And I have 2 issues with it: 1)x-axis labeling shows every single time point making it very crowded, is there a way to adjust the ticks interval on the heat map? I have tried generating a vector for x-axis labeling replacing the values that I want to hide with NaN, but it gives me an error; 2)I need to draw a vertical line (at a given x-intercept) that will indicate the stimulus application and will visually segregate samples in the heat map. None of the line functions that can be used with regular plots seem to work for the heat map (I have tried refline, vline). Attached is the sample of my heat map.