Show every failure on large control charts

Control charts with more than 500 points drew only one point in N, so most failures and spikes were missing from the chart even though the statistics counted them.

• Fixed the control chart above 500 points: every failure is now drawn, along with the lowest and highest value of each time interval, so spikes and drifts stay visible.
• Fixed out-of-spec counts and limits when several measurements with different specs are selected.
• Improved curve measurements: failed curves stay on the chart and are highlighted, even beyond the 100 most recent curves.