How often do you use the gridlines on a chart to read the data?
Not very often.
And yet there they are, prominently, when you plot your data with most graphing applications. I've said this before, and I will say it again: plotting data in a graphing application like Excel should be your first step in the data visualization process, not your last!
Gridlines typically act as nothing more than clutter, unnecessarily competing for attention with your data. Don't let them. In the event that gridlines are important for being able to read the data you are presenting, push them to the background by making them a light shade of grey. In most cases, I'd argue that your audience isn't going to make use of the gridlines at all. If this is the case, remove them completely.
Let's see what this looks like in practice through the chart progression below.
The first chart is what I get when I plot my data in Excel (using my mac).
In the second chart, I stripped out a bit of clutter by eliminating the chart border and reducing the labels and tick marks on the x-axis. I also pushed the axes and gridlines to the background by making them grey and tied the title of the graph visually to the trend line by making the title the same shade of blue. I justified the graph title and y-axis title at upper leftmost because in Western cultures most people read from left to right, top to bottom; this makes it so the audience encounters how to read the graph before they get to the actual data. This is looking better, right? The data stands out more than in the initial version, where there was no visual hierarchy to help direct our attention.
In the final graph, I removed the gridlines altogether. Note that the data stands out the most in this version, because it isn't competing visually with the gridlines for your attention.
The lesson is this: if your audience isn't going to use gridlines to read the data, get rid of them! At the very least, push them to the background. At best, they aren't particularly helpful. At worst, they distract from your data. Don't let your visuals fall victim to this unnecessary graphing application clutter!
Data source: http://seer.cancer.gov/