Setting `Chart.has_title = False` doesn't work with doughnut

Hello! Thanks for the work on this project 🙌

This is with python-pptx@0.6.10. Repro

chart_data = ChartData()
chart_data.categories = ['Yes', 'No']
chart_data.add_series('Series 1', (42, 24))

for chart_type in [XL_CHART_TYPE.PIE, XL_CHART_TYPE.DOUGHNUT]:
    for has_title in [True, False]:
        slide = presentation.slides.add_slide(presentation.slide_layouts[1])
        placeholder = slide.placeholders[13]
        chart = placeholder.insert_chart(chart_type, chart_data).chart
        chart.has_title = has_title

[scanny]

Should be fixed in release v0.6.13 later today.

[scanny]

Okay, so here’s the logic that I think makes sense of this behavior. Just to recap, in a chart (of at least certain types) that has only one series, the name of the first series appears as the title of the chart without you actually asking PowerPoint to do that.

Leaving aside the question of whether this behavior is justified, the reason why this might be reasonable is that the title can substitute for the legend in this single-series case.

Generally, a legend maps a colors or symbol to a series, so you can tell which series is which. In the single-series case, you still need the name (what the values represent) but you no longer need the color-patch because there’s only one, or because the series uses a different color for each value (bar, segment) (common with pie and I expect doughnut charts, but also an option with bar charts).

So a useful option in this single-series case is to turn the legend off and let the title explain what the chart is representing. Anyway, opinions could differ on whether that’s “helping”, but at least it makes sense to me now why they might have done this on purpose.