I was trying to plot a regression line with gnuplot, but instead to plot a straight line plot a line full of bend.I printed it in png format.Here a pic.
Given that the original question asks about why it's curved (and not about how to 'reduce the low-resolution segregation-ness' which is now being asked in comments):
The lines aren't actually curved. It's an optical illusion as a result of the data points being around the line, close enough to mess with your perception of the actual state of the line - it's really actually straight.
An example of this is seen in the U&L Meta post on the Community Promotion Ads - this post on the ad list shows how the lines are perceived as curved, when they're really actually straight.