I fixed the bug that caused the x-axis to be reversed.

I realized that the light wasn't fully coherent when it reached the diffraction grating.  (The issue can be seen in the middle of the spectrum image; there is white where there should be green and blue.)  To fix this, I made a thinner slit and moved the LED back.

I decreased the resistor from 1 k ohm to 330 ohm.  As expected, the LED is now brighter.

The holder for the diffraction grating was slightly wobbly.  I glued it to the base.  The glue is still drying; I plan to perform new tests this weekend.