New Graph Type - Text

Add a new graph type that simply displays the value and unit.

For example, a text graph for throughput using the "last" function might display something looking like "34.32 Mbps", with the text size being relative to the size of the graph.

The thought of a "text" graph is also mentioned in ZBXNEXT-5476, where it could be useful in displaying the SUM of the history of an item.