Draw a mapping diagram and graph with a visualization for $f$ on the graph and a mapping diagram for $x=0$ and $x_1=1$ or use the diagram created with GeoGebra to explore further.

Mapping Diagram for $ f (x) = 2x + 1$ $x=0,x_1=1$ |
Graph for $ f (x) = 2x + 1$ |

On the GeoGebra figure you can move either $x$ or $x_1$ on the mapping diagram.

Both these visual features are indicative of