I would like to use a line chart but I cannot decide if I should add markers to the lines or not.
What is the purpose of placing markers?
While it's not always the case, the most common reason is that markers represents the data that has been entered/registered, while the lines represent the trend or "traveling" within an X period of time where data goes from N to N', N'' and so on.
Edit: see example below: Feb had 20, yet it's impossible to tell if there was data for Feb or if the line passed thru Feb by going from ene to mar. Or in other words: We want to visualize data, yet we can't give proper information and leave data as ambiguous (which is the exact opposite of what we should do)
Image Source: Uxpanol.com
When you have peeks and valleys, we can assume the vertex is a data point. However, let's assume data goes always in more or less the same direction. For example, it goes always up. Without the data points, you won't be able to visualize whether the specific axis has data or not. Just imagine we have day1, day2, day3, day4 on the X axis. Then 100, 200, 300, 400 on the Y axis. Without data points, you could have 2 scenarios. We have data for all 4 days, or 3 days, or just day1 and day 2 going from 100 to 400, with day 2 and 3 registering no data. You can see this woudn't make any sense
And that's why you add data points: for increased visualization.
It depends from the use cases you expect this graph to be used. If it is very importand for users to know exactly which are the data points, then you must add them. But the most typical use case for graphs does not require data points to be explicitly displayed, and in this case you can remove them to increase data-ink-ratio.
Additionaly consider removing the grid lines.
