White spots in 2D contour drawing

Dear All,

I wrote a program to read in the data from a text file into a 2D histogram and then display the histogram as a contour. However, I always see some areas (or spots) in the drawing with white colors. In addition, I noticed that these strange areas change when the bin of contour changes.

I attached my code and the data file. Please help me.

Line scan spectra 2 open aperture.txt (489 KB)
2dplot.C (1000 Bytes)

Use the option COLZ instead of CONT