Summary:
The current scheme violates Runge-Kutta condition on error O(h^4) when dy is too high. This leads to visible shifting and discontinuities on the plots of integrals for e^x^2, e^abs(x), etc.

BUG: 341256

Test Plan:
1. Compile and install KmPlot.
2. Create the Cartesian plot "f(x) = e^x^2".
3. Switch to the "Integral" tab and tick the "Show integral" item.
4. Try to change the scale (Ctrl+mouse wheel). The integral curve should be plotted as expected (no discontinuities, no extra lines on Ox).

f(x)=e^x^2 and its integral

Before the patch:
{F7764991}
After the patch:
{F7764992}

Reviewers: #kde_edu

Subscribers: aacid, cfeck, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D24972