Mean of a practical distribution

Question

I have a graph with an asymmetrical distribution (spectral response for some sensor). The graph is plotted as efficiency values versus vavelength. I must determine the median wavelength. Help please, my statistics is so rusty !
I have determined the mean value of the efficiency - the values on the y-axis, and considered selecting the wavelength (x-value) corresponding to that value - but that doesn't seem to give me a relevant answer, and it is not even close to the center of the plot (even in the case where it should be). I thought of cheating and just getting the median of the graph - but the graphs are weird. In one case, the graph has a fairly rectangular shape (rise, sort of plateau but with mountains and valleys, then fall), in another case, there is a skewed peak on the left and another tiny bump on the right.
I would appreciate any suggestions ! Thank you !

Answer 1

I was eventually able to find the mean as $\dfrac{\sum (p_i x_i)} { \sum (p_ i)} $ .