  Context. Studying the spots of late-type stars is crucial for distinguishing
between the various proposed dynamo mechanisms believed to be the main cause of
starspot activity. For this research it is important to collect observation
time series that are long enough to unravel both long- and short-term spot
evolution. Doppler imaging is a very efficient method for studying spots of
stars that cannot be angularly resolved. Aims. High-resolution spectral
observations during 1994-2017 are analysed in order to reveal long- and
short-term changes in the spot activity of the FK Comae-type subgiant HD
199178. Methods. Most of the observations were collected with the Nordic
Optical Telescope. The Doppler imaging temperature maps were calculated using
an inversion technique based on Tikhonov regularisation and utilising multiple
spectral lines. Results. We present a unique series of 41 temperature maps
spanning more than 23 years. All reliable images show a large cool spot region
centred near the visible rotation pole. Some lower latitude cool features are
also recovered, although the reliability of these is questionable. There is an
expected anti-correlation between the mean surface temperature and the spot
coverage. Using the Doppler images, we construct the equivalent of a solar
butterfly diagram for HD 199178. Conclusions. HD 199178 clearly has a long-term
large and cool spot structure at the rotational pole. This spot structure
dominated the spot activity during the years 1994-2017. The size and position
of the structure has evolved with time, with a gradual increase during the last
years. The lack of lower latitude features prevents the determination of a
possible differential rotation.
