Too many elements in my SVG

Hello,

I am using C3 following options and 1500 points:

```
tooltip: {
show: false,
},
zoom: {
enabled: false,
},
transition: {
duration: 0,
},
point: {
show: false,
},
```

In DOM inspector I see that there are a lot of `` with `class=c3-event-rect c3-event-rect-N`. What is their purpose? I see that NV3D charts don't have such event rectangles.
Also there are lot of circles with zero opacity (why do we need them with zero opacity?) and `class=c3-shape c3-circle`. IMO, they all are useless.
So, the issue here that generating `O(N)` SVG elements (unneeded) consumes CPU, and should be avoided when possible.