Irregularities in tone generated using FFT data
The closer a synthesized frequency gets to half the sample rate, the better the reconstruction filter ones needs to reproduce a pure waveform.  It looks like your plotting application has a poor reconstruction (upsampling to plot points) filter, causing the resulting plot to look irregular when the signal frequency gets much above about 1/8th the sample rate.  A graph using polynomial, spline, or Sinc interpolation for the plot points would likely look much cleaner.