def transform_timeseries_data(timeseries, start, end=None):
    """Transforms a Go Metrics API metric result into a list of
    values for a given window period.

    start and end are expected to be Unix timestamps in microseconds.
    """
    data = []
    include = False
    for metric, points in timeseries.items():
        for point in points:
            if point['x'] == start:
                include = True
            if include:
                data.append(point['y'])
            if end is not None and point['x'] == end:
                return data
    return data