import matplotlib
import matplotlib.pyplot as plt
import numpy as np


def LegendVertical(Ax, Rotation=90, XPad=0, YPad=0, **LegendArgs):
    if Rotation not in (90,270):
        raise NotImplementedError('Rotation must be 90 or 270.')

    # Extra spacing between labels is needed to fit the rotated labels;
    # and since the frame will not adjust to the rotated labels, it is
    # disabled by default
    DefaultLoc = 'center left' if Rotation==90 else 'center right'
    ArgsDefaults = dict(loc=DefaultLoc, labelspacing=4, frameon=False)
    Args = {**ArgsDefaults, **LegendArgs}

    Handles, Labels = Ax.get_legend_handles_labels()
    if Rotation==90:
        # Reverse entries
        Handles, Labels = (reversed(_) for _ in (Handles, Labels))
    AxLeg = Ax.legend(Handles, Labels, **Args)

    LegTexts = AxLeg.get_texts()
    LegHandles = AxLeg.legend_handles

    for L,Leg in enumerate(LegHandles):
        if type(Leg) == matplotlib.patches.Rectangle:
            BBounds = np.ravel(Leg.get_bbox())
            BBounds[2:] = BBounds[2:][::-1]
            Leg.set_bounds(BBounds)

            LegPos = (
                # Ideally,
                #    `(BBounds[0]+(BBounds[2]/2)) - AxLeg.handletextpad`
                # should be at the horizontal center of the legend patch,
                # but for some reason it is not. Therefore the user will
                # need to specify some padding.
                (BBounds[0]+(BBounds[2]/2)) - AxLeg.handletextpad + XPad,

                # Similarly, `(BBounds[1]+BBounds[3])` should be at the vertical
                # top of the legend patch, but it is not.
                (BBounds[1]+BBounds[3])+YPad
            )

        elif type(Leg) == matplotlib.lines.Line2D:
            LegXY = Leg.get_xydata()[:,::-1]
            Leg.set_data(*(LegXY[:,_] for _ in (0,1)))

            LegPos = (
                LegXY[0,0] - AxLeg.handletextpad + XPad,
                max(LegXY[:,1]) + YPad
            )

        elif type(Leg) == matplotlib.collections.PathCollection:
            LegPos = (
                Leg.get_offsets()[0][0] + XPad,
                Leg.get_offsets()[0][1] + YPad,
            )
        else:
            raise NotImplementedError('Legends should contain Rectangle, Line2D or PathCollection.')

        PText = LegTexts[L]
        PText.set_verticalalignment('bottom')
        PText.set_rotation(Rotation)
        PText.set_x(LegPos[0])
        PText.set_y(LegPos[1])

    return(None)


Fig, Axes = plt.subplots(1, 3, constrained_layout=True)

Axes[0].bar(0, 10, label='First')
Axes[0].bar(1, 20, label='Second')
Axes[0].bar(2, 30, label='Third')

Axes[1].plot([1,2,1,2], label='First')
Axes[1].plot([3,4,3,4], label='Second')
Axes[1].plot([5,6,5,6], label='Third')

Scatters = [(np.random.uniform(-0.5,0.5,(2,10))+_).tolist() for _ in range(3)]
Labels = ('First','Second','Third')
for S,Scatter in enumerate(Scatters):
    Axes[2].scatter(*Scatter, label=Labels[S])

for Ax in Axes:
    Ax.set_xlim(-2,4)
    LegendVertical(Ax, 90, XPad=-45, YPad=12)
    # # or
    # LegendVertical(Ax, 270, XPad=-45, YPad=12)

Fig.savefig('VertLeg.png')
plt.show()