def plot_biocomp(ax, yc, yh, y_rm1, y_rm2, y_rm3):
    """
    Plot characterization of biomass sample and calculated reference mixtures
    as mass fractions, dry ash-free basis.

    Parameters
    ----------
    ax : Axes
        The Matplotlib axes from a figure
    yc : float
        Mass fraction of carbon in biomass, dry ash free basis [-]
    yh : float
        Mass fraction of hydrogen in biomass, dry ash free basis [-]
    y_rm1 : array
        Mass fraction of reference mixture RM1 where y_rm1 = [yC, yH, yO]
    y_rm2 : array
        Mass fraction of reference mixture RM2 where y_rm2 = [yC, yH, yO]
    y_rm3 : array
        Mass fraction of reference mixture RM3 where y_rm3 = [yC, yH, yO]

    Returns
    -------
    ax : Axes
        The Matplotlib axes for the plot figure
    """

    ax.plot(yc, yh, '^', label='biomass')

    ax.plot(0.4444, 0.0617, 'o')
    ax.annotate('cell', xy=(0.4444, 0.0617), xytext=(-10, 6), textcoords='offset points')

    ax.plot(0.4545, 0.0606, 'o')
    ax.annotate('hemi', xy=(0.4545, 0.0606), xytext=(-10, -12), textcoords='offset points')

    ax.plot(0.6977, 0.0543, 'o')
    ax.annotate('ligc', xy=(0.6977, 0.0543), xytext=(-10, -12), textcoords='offset points')

    ax.plot(0.6055, 0.0642, 'o')
    ax.annotate('ligh', xy=(0.6055, 0.0642), xytext=(-10, 6), textcoords='offset points')

    ax.plot(0.5687, 0.0521, 'o')
    ax.annotate('ligo', xy=(0.5687, 0.0521), xytext=(-10, -12), textcoords='offset points')

    ax.plot(0.5921, 0.0395, 'o')
    ax.annotate('tann', xy=(0.5921, 0.0395), xytext=(-10, -12), textcoords='offset points')

    ax.plot(0.7634, 0.1116, 'o')
    ax.annotate('tgl', xy=(0.7634, 0.1116), xytext=(-6, -12), textcoords='offset points')

    x = 0.4444, 0.5921, 0.6977, 0.7634
    y = 0.0617, 0.0395, 0.0543, 0.1116
    ax.fill(x, y, '0.8', alpha=0.5)

    ax.plot(y_rm1[0], y_rm1[1], 's', label='rm1')
    ax.plot(y_rm2[0], y_rm2[1], 's', label='rm2')
    ax.plot(y_rm3[0], y_rm3[1], 's', label='rm3')

    x = y_rm1[0], y_rm2[0], y_rm3[0], y_rm1[0]
    y = y_rm1[1], y_rm2[1], y_rm3[1], y_rm1[1]
    ax.plot(x, y, 'k:')

    ax.grid(color='0.9')
    ax.set_axisbelow(True)
    ax.set_frame_on(False)
    ax.set_xlabel('Carbon mass fraction, daf basis [-]')
    ax.set_ylabel('Hydrogen mass fraction, daf basis [-]')
    ax.tick_params(color='0.9')
    ax.legend(frameon=False)

    return ax