Should each month tick be at the first day of month, or every 30 days?

Question

I am making a chart with a time axis. This axis can have yearly, monthly, dayly, or hourly ticks, depending on how big the timespan is.

For monthly ticks, I'm in doubt as to where should the ticks be exactly. I can think of 2 options:

1. Each tick is on the first day, at 00:00 hs, of that month. The problem is that since months have unequal durations, the ticks won't be evenly spaced and that might look bad.
2. The first tick is the first day, at 00:00 hs, of the first month that appears. Each other tick is exacly 30 days after. For 12-20 months (the most I would show), I don't think the ticks will be too far off the actual first day. However, it still bothers me that they won't be exacly on the first day.

Since there are millions of websites with monthly charts, surely there is a concensus on this! What's the best option? Or maybe the most commonly used option?

Answer 1

For each type of time axis you implement, the tick will represent exactly one unit of value that you want to break it down into, unless you are putting in major and minor tick marks to break the single month unit into days.

I assume that you are aggregating your data by only one period type at a time, that means each graph should only have one type of value on the time axis. It seems like you would only have the problem you are talking about if you plot a graph that is based on data aggregated by day, and you want to display it by month. Instead you should aggregate the daily data into months and display it accordingly.

Alternatively, you can display the daily data and have each tick represent the value for each day, and provide tick labels at even intervals without it being aligned to specific months. An example is the data for Google Analytics where dates are labelled at even intervals apart.