There isn’t necessarily a “correct” way to style your axis lines or gridlines in our data visualizations. It is worth noting, however, that how we draw our axes can impact how our readers perceive specific values. In the case of a line chart, for example, we are probably inclined to think that the bottom of the vertical axis is zero. In some cases—especially where the data series are both positive and negative—this can be especially important.
Take the chart above that shows the year-to-year percentage-point change in health care spending as a percentage of GDP (taken from my book). In this case, because the zero baseline is not clearly delineated, it’s not immediately clear that there are any spending declines over the period. By just darkening that axis line a bit–as in the version below–it is more evident that there are three years when health care spending as a share of GDP declined year-over-year.
Choosing where to place and how to style your axis lines can be an important part of data visualization design. You can develop your own style guide to set rules for how you want the axes to appear and be careful to make decisions like these.
I talk more about these and other related issues in my new book, Better Data Visualizations: A Guide for Scholars, Researchers, and Wonks.