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A histogram is just a special kind of column graph, for displaying certain kinds of data.
A histogram has the different data values on the horizontal axis, and the frequency
always goes on the vertical axis. A histogram always has the frequency on the vertical axis.
The centre of each column is labelled with the relevant data value. There are no gaps
between columns, but there is a half-width gap at the left and right sides of the data.
The reason for leaving that gap is that sometimes we draw on top of the histogram a frequency
polygon. The frequency polygon is made by simply joining together the centres of the
top of each column with a line. And for that to work at the left and right edges, we need
at least a half a column width extra space, to get that line down to the zero frequency
value, down to the horizontal axis on both sides where the centre of the next column
would have been.
But what about grouped data? How do we draw a histogram for grouped data?
Well, you can still draw it the same way. The centre of each column is labelled with
the class centre, the middle value of each group or class. But there is an alternative.
You can instead, if you prefer, label the edges of the column with the boundary values
between the groups, like this. You can choose. Usually the boundaries would be labelled,
like the one on the bottom here, for continuous data.