The trends section automatically shows the most relevant time series on a tag. Below the trend, you can select other time series on the tag. You can also use the compare function in the top right corner to compare with time series from other tags.
There can be a short time delay when you view trend data. You can see the time difference between "now" and the sensor timestamp of the last data point in the brackets in the legend.
The default time window when you open the trend page is 1 day. The default y-axis setting is computed based on last years values, disregarding any out-of-bounds values (typically when sensors fail).