QCustomPlot: how to convert data index to axis value and vice versa

[sitesv]

Hi!
Trying to explore this title for a few hours...
Please, explain how to get a visible x-axis value for the data index?
For example:
QVector<double>x,y;
x - has a values from 0 to 1000.
xAxis has a Range values from -17000 to 17000.

Don't understand, how to get xAxis value by a value x? What a visible value of x = 223?

[sitesv]

ui->widget->graph(0)->dataMainKey(index);