Graphites storage schema in combination with Icinga is tricky sometimes. Configuring a storage schema in graphite that does not match the check interval can lead to a state where Graphite can not properly aggregate the metrics. Hence, you will have (large) gaps in your graphs.
In our docs we have a recommended storage schema for Graphite: https://icinga.com/docs/icinga2/latest/doc/14-features/#graphite-carbon-cache-writer
Still, if you have different check intervals, you have to make sure that you have a storage schema configured with a matching pattern and retention.