planetmaker, in their answer, correctly points out the existence of the construction etw. ist zu tun, meaning "something is to be done", "something has to be done", which also exists in Latin - for instance in the famous quote Ceterum censeo Carthaginem esse delendam - "Furthermore, I consider that Carthago has to be destroyed".
With verbs like tun or erledigen, the construction etw. ist zu + infinite verb expresses a necessity, the fact that something has to be done:
- Was ist jetzt zu tun? - "What has to be done now?"
- Die Hausaufgaben sind zu erledigen - "The homework has to be done"
However, most often this construction does not express a necessity, but a possibility (often negated), the fact that something can be done. These are some examples for this:
- das ist zu schaffen - "that can be done"
- das Spiel ist nicht mehr zu gewinnen - "the game is not winnable any more"
- das Raumschiff ist nicht zu betreten - "the spaceship cannot be entered"
- das ist mit mir nicht zu machen - "You cannot count on me doing this"
In constructions with verbs of perception, this construction means that something is perceivable:
- Vom Balkon ist das Meer zu sehen - "You can see the sea from the balcony"
- Die Musik war noch in der Ferne zu hören - "You could hear the music even in the distance"
And this applies to your example as well:
- Es war ein stetiger Anstieg bei den Verkaufszahlen zu verzeichnen. - "One could notice a steady increase of sales numbers."