Re: Some thoughts about histogram classes in root [LONG MESSAGE]

From: Eddy Offermann (eddy@rentec.com)
Date: Wed Feb 16 2000 - 17:21:14 MET


Hi Damir and Rene,

In the past , when trying to display time series, I hit exactly
this problem. Time series have as the running variable for instance
business day. (2/16/2000 = 1751137). I solved my problem
(as suggested here by Damir) by shifting my data in the time-series displaying
module such that the origin coincided with the first data point
and adding this shift back when plotting the X axis labels
(which are date strings).

Eddy 

> Rene Brun wrote:
> >   - Damir Buskulic has raised the question of precision in TAxis
> > (float).
> >     Should we promote float to double in TAxis ? If yes, TH1
> > constructors
> >     must be changed accordingly.
> 
> Hi Rene, Vincent and others,
> 
> For that question, I don't know if the this is the best way to go.
> Clearly, you shouldn't need more than a float to describe an axis. Going
> to double will probably solve the problem but not in an definite way,
> though it's probably necessary. The problem is that all the coordinates
> (so the axis) are relative to an origin that may be very far from the
> viewing point, so one has to compute differences between big numbers,
> which lead to the behaviour we've seen :
> 
>                                                --------------
>                                                |            |
>                                                |pad, axis...|
>                                                |            |
>                                                |            |
>   x----------------------------------------------------------
>  origin                                          6            6
>   0                                            10           10 +1
> 
> A solution should ensure that all the calculations made for drawing the
> axis, histos, etc... never lead to such differences. It could be to have
> a shift or reference point at the beginning of the pad. All calculations
> would be made refering to this point and only at the end would one add,
> for plotting numbers, the distance to the origin (add in double
> precision obviously). This certainly leads to changes in TAxis but also
> in TH1.



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