Histogram aggregation produces one plot per ensemble member

### Describe the bug

When running histogram aggregations the finest grading you can get for a total histogram across all times is per ensemble member rather than for the full ensemble.

### How to reproduce

Steps to reproduce the behaviour:

1. Select a histogram plot, aggregation over all.
2. See that a histogram is produced for each member, not the overall enesmble.

### Expected behaviour

A single histogram using all ensemble members is created.

Reproduce the issue by selecting a histogram plot with aggregation set to all, then trace the histogram aggregation path to find where ensemble members are separated. Done means the aggregation produces one histogram containing all ensemble members rather than one plot per member.

Written by the indexing model from the issue text.