Problem with "static min" and "static max"

[#1]

Good day,

I want to make a graph that displays the absences of person on a time line.

I have a table with :
- id of the absence
- person
- start date
- end date

To aim this, I use a bar chart stacked.

My dimensions are :
- Person
- ID of the absence

My expression is

[End date] - [Start date]

To have the correct space between 2 stacked bars, I use the bar offset :

if(
 isnull(above([End date])),
 0,
 [Start date] - above([End date]))

To accomplish what I want, I supposed that I needed to set the first offset to "[Start date]" and change the "static min" and "static max" to "min([Start date])" and "max([End date])".

Unfortunately, the "static min" and "static max" don't behave like I thought...

If I look my values between 40 and 80, I see this :

- 1 value near 45 for Bob
- 1 value near 60 for Bob
- 1 value near 70 for Toto

So, if I set "static min = 40", "static max = 80", I thought I would have a zoom on this part of the graph.

Here is what I have :

- 1 value near 52 for Bob
- 1 value near 68 for Bob
- nothing for Toto

Why is there a such difference ?

Can anyone tell me what I've done wrong ?