They say that a picture is worth a thousand words well this one is worth 4. "colour blind physics teacher". It's not that I can't see the difference it's just that colours aren't so obvious to us colour blind people. I was making these while my students were doing the EMF and internal resistance practical. They got good results for EMF but were way out for the resistance. The problem was that I got them all so worried that they were going to blow a fuse in one of the multimeters that they all chose very safe resistors with huge resistance. That meant that the scale on their graph was huge and any tiny variation in the best fit line led to a massive change in the intercept even though it still looked very close to the origin. Something to mention in the evaluation I guess.