I'm not 100% sure what projection a Jeppesen chart uses- sounds as if it is a Lambert projection.
On a Lambert, the great circle track between any two points on the chart is straight line on that chart. If you check such a line, you will see the angle at which it cuts each successive line of longitude changes progressively, showing that mapped onto the surface of the earth the track will be curved.
Does that help?
You can spot a Lambert projection because all the lines of longitude are straight lines pointing to (& meeting if projected to) the apex at the Pole. Lines of latitude will be arcs of a circle.