# Secondary Axis

Quantities that share the reaction coordinate but not the units - bond lengths against energy, say - can be drawn on a right-hand y-axis by passing `secondary` to `plot()`.

It takes a dict of `{label: [values]}` on the same x-indices as the primary series, folded into the same legend. Set `y2={'label': ...}` so the right-hand scale says what it is.

```python
plotter = ReactionProfilePlotter(
    figsize=(7.6, 4),
    curviness=0.0,
    labels=False,
    energy="E",
    square=True,
    x_label="reaction coordinate  λ",
    legend={"outside": True, "anchor": 1.22},
    y2={"label": "bond length (Å)", "colors": "plasma"},
)
plotter.plot(
    {"force field": [0.0, 5.2, 12.4, 3.1, -1.2], "g-xTB": [0.0, 4.4, 10.9, 2.0, -2.5]},
    secondary={
        "C1-O3 (breaking)": [1.43, 1.62, 2.10, 2.85, 3.30],
        "C1-O14 (forming)": [3.20, 2.75, 2.05, 1.55, 1.42],
    },
    filename="../images/profile24",
)
```

```{image} ../images/profile24.png
:height: 300px
:alt: Secondary axis
```

## Styling the two axes

A style key set on the plotter applies to **both** axes. The same key inside `y2` overrides it for the right-hand axis alone, so the two can be styled independently:

```python
ReactionProfilePlotter(
    curviness=0.0,
    point_type="bar",                 # both axes
    y2={
        "point_type": "dot",          # right-hand axis only
        "curviness": 0.42,
        "colors": "plasma",
        "linestyle": "solid",
        "desaturate": False,
    },
)
```

Any of `colors`, `curviness`, `line_width`, `marker_size`, `point_type`, `bar_length`, `bar_width`, `connect_bar_ends`, `desaturate`, `desaturate_factor` and `dash_spacing` can go in `y2`, plus two of its own: `label` for the right-hand axis label, and `linestyle`, which defaults to `'--'` here so the second scale reads as separate. Set `'solid'`, or any matplotlib spec - see [Plotting Customisation](customisation.md).

## Legend

The `legend` dict is passed straight to matplotlib's {meth}`~matplotlib.axes.Axes.legend`, plus `outside` and `anchor`. With a secondary axis, `anchor` needs about `1.22` to clear the right-hand axis and its label; `square=True` then stops the axes being squashed to make room. See [Plotting Customisation](customisation.md) for the full set.

```{note}
`axes` governs the secondary axis too: `axes='None'` draws no right-hand spine or ticks.
```