'Save ggplot to a .png file.
Parameters
filename : string
filepath to save to
width: int, float
width of the plot in inches
height: int, float
height of the plot in inches'

def save(self, filename, width=None, height=None, dpi=180):

    self.make()
    (w, h) = self.fig.get_size_inches()
    if width:
        w = width
    if height:
        h = height
    self.fig.set_size_inches(w, h)
    self.fig.savefig(filename)
