Generate a kind of propability (color) plots from many single result curves

i want to create color plots from multiple curves so that dense looking curves are visually easy to understand as shown in picture above. i used histc to get number of points falling into each bin at each frequency and later use contourf to plot. however the outcome does not look very smooth. Also if all points fall i one bin and the bin size is big then the color plot look shifted. Any better way to do this. Thanks.