I assume your using the design view to build your chart
First, you need to know that there is the chart and there are series.
You place the chart object on your webform first, and size it, and then paint it, using the properties F4. When you have designed your chart, then you can build series, which are the data points. So start with adding manual data points first to learn how to build a series.
When you chart looks good, now you can attach your data to the chart.
So now you go to the code behind page, and make a reference to the chart, in which you create a new series, and then add the series to the chart.
The link below shows how to write code for series, and attach it to the chart.