После создания диаграммы вы получаете доступ к каждой строке по серии, которую представляет эта строка:
graphic_frame = slide.shapes.add_chart(..chart_data,..)
# ---graphic frame is the shape containing the chart, not the chart itself---
chart = graphic_frame.chart
# ---get the desired series from the chart---
series = chart.series[0]
# ---series.format is a ChartFormat object, which in turn provides access to
# the LineFormat object for that series---
line = series.format.line
# ---change the line style, color, etc. the same way as for any other line---
line.color.rgb = RGBColor(255, 0, 0)