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
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.