A bump chart is rank over time: one line per competitor, plotted on a
value axis drawn upside down so first place sits at the top. amCharts has
no bump series — the chart is ordinary LineSeries and an
AxisRendererY with inversed: true — so the
adapter reads the inversion together with the values, which have to be a
genuine ranking: whole places, one competitor per place per round, and
somebody in first.
That distinction is the whole point. A rank is not a magnitude, so read as a line chart the leader would be sonified as the lowest note on the chart and a team climbing the table would be heard falling, on every move, with nothing to say the reading was upside down. MAIDR's bump trace inverts the pitch, announces the places gained or lost alongside the rank, and offers rank-gained / rank-lost rotor units (Alt+Shift+Up/Down) that jump between the rounds where something actually moved.
The same chart drawn the right way up. Nothing about the numbers says
which way the axis runs, so the bump: true option is what
says these lines are places; the values still have to read as a ranking
before MAIDR announces them as one.