When you report metrics at regular intervals, we can detect gaps if you set the Period in your Metric Attributes.
Some metrics are reported opportunistically though, i.e. every N seconds a measurement is sent only if there is new data. This results in a sparsely populated series. When the Period is set and Gap Detection is enabled, we render it as a set of invisible dots due to our gap detection.
With Gap Detection and the aggregation method you have three ways to render your graphs:
1) gap-detection enabled -> always render gaps
2) gap-detection disabled, aggregate by average -> connect points with straight lines
3) gap-detection disabled, aggregate by sum -> zero-fill between points