tto toggle cursor tracking.
The line at the bottom represents the values of |zn| at the location of the red crosshairs, until |zn| “escapes” (becomes greater than 2) or all the iterations are calculated. The line swirling around the graph starting from the cursor similarly represents actual values of zn, not just their absolute values. Increasing the amount of iterations increases the detail rendered, and by rendering a sufficiently large number of iterations, we may render a picture arbitrarily close to the actual, infinitely-detailed Mandelbrot set.
Any white “holes” in the set are simply unfortunate graphical errors, and should be disregarded.