I guess you could have a list of buckets representing mappable areas of the files.

Then, when you create your plottable values, you throw them in the appropriate bucket (what portion of the file they would be based on x,y), including x,y,value.

After you have a lot of them, you go through the buckets one by one:

1.Load chunk of file corresponding to bucket

2.Write all the values to that file chunk (using the x,y coord you stored along with the value)

3.Unload that chunk

4.Repeat for next bucket

This would make the writing to file more sequential instead of having to repeatedly load and unload different parts which would happen with completely random access.