Typically data charts have confusing axis labels. Some sort of title horizontally across, some sort of subtitle below it, and then a upright, flipped axis label on the y-axis, and a label on the x-axis. Too many labels, and also, I don't like to have to tilt my head to read the y-axis label.

Here is what I usually do:

• Put a slide title that tells the message that the data is supposed to show ("facebook users are getting older")
• A subtitle that describes the unit: average age of a facebook user
• Omit the y-axis label, and leave the x-axis label out all together, if the columns' base says 2013, 2014, 2015, 2016, people will figure out that it is by year