def hist_plot_encoding(plot_config):
    """ Build encoding portion of Vega plot description for a histogram plot.
    """
    encoding = {
        "x": {
          "bin": True,
          "field": plot_config.x_col_name,
          "type": "quantitative"
        },
        "y": {
          "aggregate": "count",
          "type": "quantitative"
        }
    }
    return encoding