The trace with the Focus
There are many settings that can be used to control the operation of QtChart and the appearance of the chart window. Some of the settings have one value that applies to all the traces, while others have one value for each trace, and can be set to different values for each trace.
One of the settings of primary importance is call the Focus. For settings that can have different values for each trace, the Focus determines which trace the setting currently applies to.
For example, the settings for the horizontal and vertical scaling for the chart window each have one value displayed at a time. For the horizontal scale setting, there is just one value that applies to all the traces at once. But the vertical scale setting can have a different value for each trace. So if you look at the vertical scale value, or change it, you need to know which trace the value applies to. Which is were the Focus comes in.
One of the settings in QtChart is the color associated with each trace. Of all the settings that can have different values for each trace, this is the only one where you can see the value for each trace. And it is this value that is used to quickly determine which trace has the focus.
Setting the Focus
The upper right portion of the QtChart window contains a grid of 8 colored squares and, for each trace that is in use, the name of the data element being charted.
To choose which trace has the Focus, you simply click on the area just to the right of one of the colored squares (where the name of the data element appears, if a name has been specified). You will know it worked when the color of the rectangle just above the Mode setting changes to match the color of the trace you selected.
The controls to the left and below the colored rectangle each show settings for the trace that currently has the focus. Any changes you make to those settings will affect only the trace with the focus.
Also, left and right clicks within the chart window are applied to the trace with the focus (click here for more info).