This graph shows the daily traffic on your site over a time period of your choice (30 days by default). You can change that period by selecting a different date range from the top of the page.
Each bar represents one day, and the bar's height represents the total number of [ visitor sessions ] on your site during that day, regardless of whether these were [ first time visitors ] or [ return visitors ] (the exact number can be read at the bottom of each bar or by hovering your mouse over the bar).
Note that Saturdays and Sundays are represented by slightly lighter colored bars
If the period you are looking at is greater than 60 days, a red line will appear superimposed to the graph. It represents the 28-day rolling average of your daily traffic. Since it smoothes out the daily variations it gives you a better idea of your long-term traffic trends.