Quick explanation: line charts are for continuous data, bar charts are for discrete data. The numbers of visits per day is a classic example of discrete data and therefore a bar chart would be the right chart.
The current line chart falsely indicates that their are more values between the displayed point. But, for instance, if there were 10 visits on Monday and 20 visits on Tuesday, than you can't say that there were 15 visits in the night between both days or something.
I just investigated in how much work it would take to fix this bug. Fortunately, it can be done in seconds. We just need to change the $graphType to "bar" in line 21 of /core/ViewDataTable/GenerateGraphHTML/ChartEvolution.php
You can see a running demo here: http://piwik.vis4.net/
I understand/agree, but I'll let Matt chime in since this has been the way since the beginning.
Out of curiousity, I increased the number of data points, and it still looked ok. (The only problem is X-axis labels.)
While I agree with the explanation, I find that the bar chart graph is more "heavy weight" in term of visual impact, I find there are too many dark pixels and it looks less nice.
Note: there is a small bug that the graph does not use 100% width.
At this stage I would prefer not to change but maybe there are ways we can make the bar chart looking nicer.... what are your thoughts?
Please comment if any feedback
Maybe an alternate chart that contains both bars for discrete values + a spline function for a continuous line. (see also #1718)
I think this is good idea. Will try to investigate into this in the future. Another idea would be to allow users to change from line to bar charts and to store the choice in the user settings.
Being able to switch between line and bar is actually implied by the current UI. A 'bar chart' icon is visible in the chart footer, while its hover text reads "vertical bar graph". Clicking on it just refreshes the line graph. This is a bug.
+1 to implement greg's idea in comment:5. Otherwise, simply remove the bar chart icon from the footer.
Good point, we could change the "Bar chart" icon for a "line chart" icon (example) for the "evolution" graphs footer.
In 967c39814c7353dc51d5f7cfcde240334e936ecb: Refs #2608, add display property to evolution graph allowing it to be plotted as a bar graph.
The bar graph looks a bit off, but this will be fixed in #3089. Closing this ticket.