Mean absolute percentage error

These waves are VERY different... This whole thing does not make much sense. 

Here is what I would do:

The fist line creates wave for short version of y1

Second line fills it up with interpolated values from y1/x1 par at x2 values. Now y2 and y1 have same number of points and are for same x values. 

Third line creates wave for the difference.

Fourth line calculates differences for each point and divides by whichever wave you consider base. And takes absolute value of that. You may need to modify this so it gives what you need. May be scale by 100 to make it %? 

Now, we need to make sure we use data only in the overlapping range of x values. Based on what you said above, you can use simply minimum 270 and 320, since both data sets start at same x value, we can use from point 0. But only up to point, where x<=320. BinarySearch(x2, 320 ) finds that point number for us. 

Therefore results can be obtained doing sum(Difference, 0, BinarySearch(x2, 320 )) - sum of the calculated Differences between points 0 (where x2=0) and point number, where x2<=320, end of the overlapping region. 

No guarantees, there may be some bugs in this. 

Now, I tried to do this, but the values in y1 and y2 are so different, it seem meaningless max of y2 is 1.5 while for y1 it is 1.6e10 :

•wavestats y1
  V_npnts= 2000; V_numNaNs= 0; V_numINFs= 0; V_avg= 5.86345e+09; 
  V_Sum= 1.17269e+13; V_sdev= 4.23897e+09; V_sem= 9.47863e+07; 
  V_rms= 7.23463e+09; V_adev= 3.59572e+09; V_skew= -0.187221; 
  V_kurt= -1.24415; V_minloc= 0; V_maxloc= 601; V_min= 0; 
  V_max= 1.64566e+10; V_minRowLoc= 0; V_maxRowLoc= 601; 
  V_startRow= 0; V_endRow= 1999; 
•wavestats y2
  V_npnts= 236; V_numNaNs= 0; V_numINFs= 0; V_avg= 1.37021; 
  V_Sum= 323.369; V_sdev= 0.656998; V_sem= 0.0427669; 
  V_rms= 1.51898; V_adev= 0.549667; V_skew= -0.232631; 
  V_kurt= -0.810713; V_minloc= 37; V_maxloc= 60; V_min= 0.293419; 
  V_max= 3.10107; V_minRowLoc= 37; V_maxRowLoc= 60; V_startRow= 0; 
  V_endRow= 235;