Support different number of values per series

[tkoskela]

Currently plotting the following gives an error because we force all series to contain all x axis values:

x_axis:
  value: "test_name"
  units:
    custom: null

y_axis:
  value: "flops_value"
  units:
    column: "flops_unit"

filters: []

series: [["system", "archer2"], ["system", "isambard-macs"]]

RuntimeError: ('Unexpected number of rows (9) does not match number of unique x-axis values per series (12)',                  test_name  flops_value      flops_unit         system
0   HPCG_IntelSKX_Isambard      38.6863  Gflops/seconds  isambard-macs
1            HPCG_Original      24.0270  Gflops/seconds  isambard-macs
2             HPCG_Stencil      51.0070  Gflops/seconds  isambard-macs
3  HPCG_IntelAVX2_Isambard      39.0427  Gflops/seconds  isambard-macs
4               HPCG_LFRic      18.5399  Gflops/seconds  isambard-macs
5   HPCG_IntelAVX_Isambard      38.7652  Gflops/seconds  isambard-macs
6            HPCG_Original      39.2325  Gflops/seconds        archer2
7             HPCG_Stencil     124.1970  Gflops/seconds        archer2
8               HPCG_LFRic      55.9839  Gflops/seconds        archer2)
Post-processing stopped

We should be able to relax this to allowing the total number of values to be less or equal the number of series * the number of x axis values