One day shows weather, not climate
The comparison above is about the Venice temperature today recorded on this exact date, year after year: it's an interesting data point, but a single day can vary a lot for purely random reasons, such as a storm system passing a few hours earlier or later, or a different wind pattern, regardless of the broader climate. It shouldn't be read as scientific proof of climate change, just a snapshot of "today versus the past".
The data comes from the ERA5 climate reanalysis (Copernicus / Open-Meteo), which estimates the temperature over the lagoon by combining satellite observations with nearby weather stations.