Plot data in ggplot2

Hi, here is a reproducible version of your data.

df <- tibble::tribble(
  ~time_s, ~dS2_mV, ~oS2_mV, ~dS2_mr_cs, ~oS2_mr_cs,
        0,    211L,    421L,       1.08,      -12.7,
      0.5,    211L,    421L,       2.16,      -25.4,
        1,    212L,    421L,       3.47,      -37.9,
      1.5,    211L,    421L,       4.55,      -50.7,
        2,    212L,    421L,       5.86,      -63.6,
      2.5,    212L,    421L,       7.39,      -76.5,
        3,    212L,    421L,        8.7,      -89.5,
      3.5,    212L,    421L,       10.2,       -102,
        4,    212L,    421L,         12,       -115,
      4.5,    212L,    421L,       13.8,       -127
  )

Here are some examples, though whether secondary axes are a good thing is up to you:

Dual axis charts in ggplot2 - why they can be useful and how to make them | Data By John (johnmackintosh.net)

Creating A Dual-Axis Plot using R and ggplot (finchstudio.io)