Hi, I want to add the capability of derivative plot function to the program. So, I followed the instruction at http://www.exelisvis.com/...i/Plot_Functions.pdf step by step successfully and added "pf_1st_derivative", and "pf_2nd_derivative" functions, but I get an error whenever using these functions. the error says they are undefined function. I get the same error even when I run them inside IDL, so I guess the problem is because of the code itself. Could you please help me resolve this problem.