Weird break in some cross-sections

**This issue is a bug report**

* **What is the current behaviour?**
A weird break appears in some cross-sections in the graphics selection window

* **If the current behaviour is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem**

1. Open the **collide.h5** example project
2. Open the graphic selection window, Click **Insert > Measurement Point > Graphic Selection**
3. Change the plane normal to **Custom Normal** and set the normal to [1, 1, 1]
4. Set the plane distance to 158.770, the cross-section should be as shown below with no breaks (first image)
5. Set the plane distance to 158.771, the cross-section shows a break (second image)

* **What is the expected behaviour?**
There should be no break

* **Please tell about the environment(s) affected:**
All

* **Other information (e.g. detailed explanation, screenshots, stack traces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc):**