If you mean the box that lists the meanings and values of all the various colors, symbols, letters, etc. found on the entirety of the chart, it's called the legend. If you mean, specifically, the line that runs from left to right across the chart, a line used in many but not all types of charts, it's called the abscissa. That term is rarely used, though. It is most often called the x-axis or the category axis.
It is called the "key".
time
Legend
The legend is the function that identifies the data marker for each series in a chart.
diagram
The Doughnut chart and pie chart, compares relative values of different categories to the whole
A bar chart.
A legend or key.
Legend
The boundary of a chart across the bottom and up the side can be the axis. It depends on the kind of charts. They are use to have the values or categories of data. In a column chart, the base of the column is at the X Axis for example.
The 5-W's are: who, what, when, where, and why.
The legend identifies the colors that represent the bars.