fid = fopen('data.prn', 'rt');
datacell = textscan(fid, '%*s%f%f', 'HeaderLines', 2);
fclose(fid);
plot(datacell{1}, datacell{2})
However, keep in mind that your screen only has a few thousand pixels, not billions, so you are going to end up with multiple data points per pixel, which will be slow and of no good visual purpose. You should consider plotting only a subset of the data, such as
plot(datacell{1}(1:1000:end}, datacell{2}(1:1000:end))
