  We study the gas emission of galaxies with active star formation, consisting
mostly of HII galaxies and Starbursts, as well as some Seyfert 2s, and
determine chemical and physical parameters. The data consist of 19 high
signal-to-noise ratio optical templates, a result of grouping 185 emission line
galaxy spectra. Underlying stellar population models were subtracted from the
templates in order to isolate the pure emission component. We analyse the
distribution of these improved signal-to-noise ratio emission spectra in
diagnostic diagrams and find that the HII templates show a smaller spread in
log([OIII]/H\beta) values than the individual galaxies, apparently due to the
population subtraction and better signal-to-noise ratio. We thus suggest the
template sequence as a fiducial observational locus for HII galaxies which can
be used as reference for models. The sequence of line ratios presented by the
HII galaxies in the diagram log([OIII}]\lambda5007/H\beta) X
log([NII]\lambda6584/H\alpha) is primarily due to the gas metallicity, of which
the log([NII]/H\alpha) ratio is a direct estimator. We also study the
properties of the Starburst galaxies and those intermediate between HII and
Starburst galaxies, which are more metal rich and sit on more massive galaxies.
We discuss the present results in the frame of a recently proposed equivalent
width diagnostic diagram for emission line galaxies and conclude that the
observed ranges in W([OII])/W(H\beta) and W(H\beta) are mostly due to the
non-ionizing stellar population contribution. W(H\beta) can be used as an
estimator of this contribution to the continuum, and briefly discuss
implications to the cosmological use of HII galaxies.
