### 2D plotting in sage looks wrong

I am a beginner using sage, and I have encountered a problem using the plot function.
Plotting a simple sine or cosine function is not very precise:

```
t=var('t')
v=plot(sin(t),-pi,pi
```

The function is clearly positive in t=-pi, where it should be zero.
The mistake becomes even more clear when setting line thickness to 99:

```
t=var('t')
v=plot(sin(t),-pi,pi,thickness=99)
```

In this case when x=0 the line is crossing 1 in the upper side and not -1 in the lower side.

This problem occurs in all functions i have encountered so far, as if the function is displayed 1 or 2 pixels off.
It's even worse when plotting arrows:

```
plot(arrow((0,0), (2,3)))
```

Here the arrow is clearly not starting in (0,0) as it should.

Is there a solution for this kind of problem?