The metric detail page showed the empty overlay ("No samples in this
range yet — check back in a minute") whenever the initial history fetch
returned zero points, but nothing ever hid it again. Live ticks then
pushed samples in, drew the chart and filled the now/peak/avg/min stats
— while the overlay stayed up, contradicting the visible data.
Make _renderChart the single authority for the overlay: hidden whenever
there are points, shown only when there are none. Live data clears it as
soon as the first sample lands; switching to a genuinely empty range
brings it back.
Signed-off-by: librelad <librelad@digitalangels.vip>
b7679bb384