What chart object is a horizontal/vertical line extending across the plot area (aiding reading values)?

This is the category axis. It’s actually the most basic X-axis on a chart that is dealing with any kind of values that has become a graph. It is usually a date axis if you are dealing with time lines. In Excel, you can change the category axis to a date axis pretty easily. However, Excel will do the hard part of choosing which part is the category axis for you. It’s the date, the item, or even the time.

The price, the temperature, the number of items, etc. will go on the Y-axis. The default is whatever the smallest difference is. If you have a span of seven days, with each day being the next chronological day, the axis on Excel will read in days.