You can choose between the stations available in the menu "Station"
Use the calendar for a specific date. It may happen there aren't any data for your picked date.
By clicking on "+" the data of a station and a specific date can be added. It will appear in the list and in the chart. By "-" you can eliminate specific data. Make sure to pick a certain dataset by clicking in the list before you remove it.
Last but not least, you can zoom by using the two handles on both sides in the overview below the chart itself.