show speed checkbox works only in the Elevation-time and Distance-time tabs about viking HOT 3 CLOSED

I do see the "Show GPS Speed" checkbox working for all five graph tabs.

Not sure what show DEM is supposed to do though.

from viking.

The point of Show GPS Speed is to display speed as reported directly in source data.
E.g. for a GPX file this is taken from the <trkpt><speed> field.

However such fields are not mandatory and typically (perhaps more so from older devices) missing.
Hence Viking almost always calculates the speed between trackpoints and uses that for display by default (this is what can be shown on the Elevation-Time Distance-Time graphs - and hence always works).
All the Garmins I've used never outputs these fields, but I think at least one app on my mobile does.

Yes you will need DEM data for the Show DEM to do something useful.
It could be disabled if you have no DEM Layer, as then there is no DEM data available.
However if you do have a layer but may be not data covering (or only partially covering) the area of track, I'm unsure what form the user feedback should take in this instance (an auto timing out popup might be good - but not easy in GTK2 - IIRC there's one in GTK3).

Perhaps the manual should explain some of this detail between 'Speed' vs 'GPS Speed'.

from viking.

What I think could already help is to have all six tabs have the checkbox named "Show GPS speed", now four tabs has it as "Show GPS speed", the other two (Elevation-time, Distance-time) has the button named "Show Speed" (without GPS).

Show DEM is indeed working once DEM data is available.

from viking.