I am writing a thesis and don't know if the charts below should have grid lines or not. Do they need the grid lines?
I would say that it is really up to you here. I don't think that the grid lines are distracting in any way, but can be helpful if you are trying to show more exact results. For example, it if it important that the scatter plot shows exact points then keep them. However, if you are demonstrating more of a general trend, then you could go without them. I'm not sure if these are your finished graphs or not, but make sure to label the color differences in the lines and provide a title. Also, is there a reason why your x-axis is on the top of the graph? If you want to keep it there and use the grid lines, try to have the numbers be above the graph instead of being slightly blocked by them.