I'm going to write a modified version of MakeGraph by mike3

[Gottfried]

But the main commands are:

func(z)

MakeGraph(WIDTH,HEIGHT,x0,y0,x1,y1,FILENAME, BLUR)

Hi James - ufff... the idea to replace the elementwise "write(' ',<color>);" by a rowwise "write(strjoin(rowvec_of_colors))" plus replacing "write()" by "filewrite()" does not gain much.

I expected something like needing only a quarter, fifth or tenth of time, but only could improve to about 3/4 (for 200x200) resp 56/95 (for 800x800) of time for the improved against your given routine (of course checked for the file-writing part alone!).            
I'm really surprised, because in my yearlong Pari/GP-experiences I *had* cases where the improvement by the better write-procedure was far better.            
Don't know what's going around; perhaps Pari/GP in its version 15.0 is already better than older versions in this hindsight - don't know. 

So unfortunately I cannot be of much help here...

Still wishing you more good good luck as before...  

Gottfried