Re: [GT] logarithmic values and scale.pm

["robert a. schmied" <ras AT acm.org>]

aloha Oliver

Oliver Bossert wrote:
> 
> Hi ras,
> 
> >i'm thinking it is caused by either plotting RSI or SMA curves on a log price
> >plot.

after more closely looking at the graphic.conf file i wish to restate the above:
i'm thinking it is caused by plotting either SMA or BOL curves on a log price
plot

> 
> When you plot the SMA the value should never be nevative as the SMA is calculated on the price and a negative price is not possible.

i agree. however, i suspect the situation was one of the boundary areas where
i was trying to access price data outside the range of the database and the
SMA calculation was generating garbage.

the problem went away when i moved the graph start well past the beginning of
data. upon review however the database starts at 1998-12-28 and the failing
graphic.pl had a start of 2001-12-31 with a default end of 120 periods, so now
i'm not sure where the problem is coming from. (note this has occurred with a
number (3-6) of my securities, with the same graphic config file. there are
3 sma curves, 200, 38 and 13 in the plot, plus a votingline that is using
a BBO indicator (that shouldn't be subject to graph scaling?)


fyi -- one of the motivations for reporting this was my feeble attempt to
point out the apparent weaknesses of the data validation testing built into
the gt code. if i understood better how the modules are coded and operate
and how to debug oo-perl i'd be willing to help with corrective patches, but
at the moment i'm just a hacker poking at the code and watching it fail.

> 
> When plotting the RSI a logarithmic scale is normally not necessary because you use the log-scale to normalize the price-movement; but the RSI is already normalized by its formula so that it is between 0 and 100.

interesting -- this might be a place to point out that documentation
of indicators should include the output value range (max limits) as well
as the default conditions and values if applicable. now watch that the RSI
docs to do just that!

> 
> Maybe you can isolate a single case where your negaitve number occurs. We can then look if it is a bug in the indicator or if the indicator is right but the number produces should not be displayed on a logarithmic chart...

i can reproduce these occurrences with my real world data, but have yet to
attempt it with the test data set. if time permits ...

> 
> CU, Olf