Crisis averted, just had to specify things properly in catalog.yml. In my case:

yml
power_spectrum_figures:
  type: PartitionedDataSet
  path: data/07_model_output/figures
  dataset:
    type: matplotlib.MatplotlibWriter
    save_args:
      format: pdf
      dpi: 300
      bbox_inches: tight
  filename_suffix: ".pdf"