Alex Velez lays out the case for tick marks:
Lately, I’ve noticed that more and more graphs don’t include gridlines. If it’s unclear, I believe this to be a positive trend. I, myself, rarely use gridlines, and often remove them when I find them in a graph I’m reviewing. But I don’t stop there.
More often than not, if a chart has gridlines, it will be lacking tick marks along the axis, and possibly an axis line as well.
Read on to see why.