def get_unique_coords(df, lat_field, lon_field):
    """
    Takes in a dataframe and extract unique fields for plotting.
    Uses dataframe group by to reduce the number of rows that need to
    be plotted, returns the lat and lon fields
    """
    unique = df.groupby([lat_field, lon_field]).size().reset_index()
    lat, lon= unique[lat_field].values, unique[lon_field].values
    return lat, lon