Embeds a dark-themed matplotlib canvas (3 stacked strip charts: theta
ist/soll, heatrate ist/soll, heater power_set/power_eff) in a new Plot
tab, sampled once per second from the latest websocket-received values.
HeaterTask now also emits a PowerSet message (the pre-discretization
target power) alongside the existing Power (effective) message, since
the GUI had no way to see the set-point side of figure 1's bottom plot
otherwise.
jens
93b5667a64