v1.6.1

Changed

• The amplitude-scale label moved from under the colorbar to under the frequency-scale
  label, and both are now prefixed (Hz LOG over Amp dB²). They could both read LIN,
  so on its own the right-hand one wasn't recognisable. Click either to cycle it, as before.
  The pair sits left-aligned in the bottom-left corner, clear of the 1 Hz ruler label.
• New README screenshot.