How to create a general plot for this problem
I think your data has three variables: customer_ID, number_of_products_sold, and feature_x. Time is not, as far as I can tell from your description, relevant.
To get all three relevant variables plotted in one figure, start with the data for a single customer and show number_of_products_sold versus feature_x as a scatter plot (removing the time aspect). This scatter plot with, say, circles for each data point, could be extended to multiple customer_IDs by overlaying scatter plots with different marker types.