To plot hourly data in Excel, follow these steps:

1. Enter your data into Excel, with the date and time in one column and the corresponding data (flow and rain in this case) in the adjacent columns.
2. Select the entire data range, including the date and time column.
3. Click on the "Insert" tab in the Excel ribbon and select the type of chart you want to create (line chart in this case).
4. Excel will automatically create a chart with the date and time on the X-axis, but it will likely be formatted as daily data. To change this, right-click on the X-axis and select "Format Axis".
5. In the "Format Axis" pane, under "Axis Options", select "Text axis" instead of "Date axis". This will allow you to plot hourly data.
6. To add a second Y-axis for the rain data, select the rain data series in the chart and right-click. Select "Format Data Series".
7. In the "Format Data Series" pane, under "Series Options", select "Secondary Axis". This will add a second Y-axis to the chart.
8. You can now format the chart as desired, including adding titles, axis labels, and adjusting the formatting of the data series.

By following these steps, you should be able to plot hourly data in Excel and create a chart with multiple data series on different Y-axes.