The answer was to use the carbon-aggregator, not the carbon-cache.
The carbon-cache will always replace the value, no matter what. If time per point is 1 second and you send more than one value within a second then the last value is what will be stored.
If you want more than one value to be kept you need to use the carbon-aggregator (running on a different port) and configure how it should aggregate the data (sum, average).