Sub/Superscript in plot in a loop

I have a for loop with counter i and I'm producing plots where I  
ultimately
want to label the axes:

"Entry a_i from P^q", where a, P and q are just letters, and i is  
the value
of the for loop counter.

I've tried various combinations of paste, expression and substitute,  
but I
can't get this to work nicely.