redraw a custom uiview when changing a device orientation

Go here to learn how to receive notifications for when the device orientation changes. When the orientation does change, just call [chartView setNeedsDisplay]; to make drawRect: get called so you can update your view. Hope this helps!

While a preferred solution requires zero lines of code, if you must trigger a redraw, do so in setNeedsDisplay, which in turn invokes drawRect.
No need to listen to notifications nor refactor the code.

Swift

override func layoutSubviews() {
    super.layoutSubviews()
    self.setNeedsDisplay()
}

Objective-C

- (void)layoutSubviews {
    [super layoutSubviews];
    [self setNeedsDisplay];
}

---

Note:
layoutSubviews is a UIView method, not a UIViewController method.

To make your chart rendered correctly when device orientation changes you need to update chart's layout, here is the code that you should add to your view controller:

- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];

    _chartView.frame = self.view.bounds;
    [_chartView strokeChart];
}

Zero Lines of Code

Use .redraw

Programmatically invoking myView.contentMode = .redraw when creating the custom view should suffice. It is a single flag in IB and, as such, the 0 lines of code prefered way. See Stack Overflow How to trigger drawRect on UIView subclass.