As Perlbotics implies, there is a fundamental disconnect between what you want to do and the way you are trying to do it. It simply does not make sense to try and display a vast amount of data on a small canvas. Perhaps you should tell us more about the nature of the data and how you want to view it so we can suggest better ways of handling the problem.

If you are plotting lots of lines then the following technique may work for you: divide the horizontal span into some number of columns (close to the number of x pixels you plan to plot). For each column generate a histogram count (with close to y pixels number of bins) of the number of lines that pass through each bin. Plot an intensity value for each bin that is a function of the bin count.