This guide explains how to create a visual like this:

The example dataset consists of a date column, a column for total visitors and another for total conversions per day.

1) Create one Mark visualising your data at the default frequency. Here we're calculating daily conversion rate via a calculated field within the visual.

2) On the same visual, plot an additional Mark with the same X values. For the Y values, we will create another custom calculation as shown.

The moving average period is defined by the window size - we used '30' here as each row in our original data represents one day.

3) To add the legend, click the 3 dots at the top and toggle on show mark as legend

See the full example canvas here.