Here is an example of vertical gradient color filled under a curve.
I would like a variation: the color filled under the curve represents a 'feature' of the variable. For example:
In the second subplot, the grey line is the feature importance of GPP (a variable), and the yellow line is the value of GPP. So, can I just plot one line representing the evolution of GPP, and fill the gap between this line and x-axis with a horizon gradient color? 'horizon gradient' means that, the deeper the filled color is, the larger the feature importance is. And the colorbar (of the filled color) involves the timing-evolution of feature importance.
By the way, I have 4 input variables and 3 output observation labels on multiple groups (4 different vegetation types, that means 4X and 3Y for each individual vegetation type). How can I plot their time evolution efficiently? (without needing to plot a HUGE amount of subplots.)