You are using discrete x-axis (bands) rather than continuous. I would do something like this.
[![enter image description here][1]][1]

{ "$schema": "https://vega.github.io/schema/vega-lite/v5.json", "width": 800, "height": 200, "data": { "values": [ { "pre_120": 10, "pre_90": 15, "pre_60": 30, "post_60": 100, "post_90": 150, "post_120": 200, "type": "Mango" }, { "pre_120": 50, "pre_90": 30, "pre_60": 45, "post_60": 90, "post_90": 140, "post_120": 190, "type": "Apple" } ] }, "transform": [ {"fold": ["pre_120", "pre_90", "pre_60", "post_60", "post_90", "post_120"]} ], "layer": [ { "mark": "line", "encoding": { "x": { "field": "key", "bandPosition": 0, "axis":{ "bandPosition":0, "labelOffset":-55}, "sort": [ "pre_120", "pre_90", "pre_60", "0", "post_60", "post_90", "post_120" ], "scale": { "domain": [ "pre_120", "pre_90", "pre_60", "0", "post_60", "post_90", "post_120" ] } }, "y": {"field": "value", "type": "quantitative"}, "color": {"field": "type", "type": "nominal"} } }, { "mark": { "type": "rule", "color": "maroon", "size": 3, "strokeDash": [6, 4] }, "encoding": {"x": {"datum": "0", "bandPosition": 0}} } ] }

[1]: https://i.stack.imgur.com/Vu8mP.png

[![enter image description here][1]][1]

{ "$schema": "https://vega.github.io/schema/vega-lite/v5.json", "width": 800, "height": 200, "data": { "values": [ { "pre_120": 0, "pre_90": 0, "pre_60": 0, "post_60": 100, "post_90": 150, "post_120": 200, "type": "Mango" }, { "pre_120": 0, "pre_90": 0, "pre_60": 0, "post_60": 90, "post_90": 140, "post_120": 190, "type": "Apple" } ] }, "transform": [ {"fold": ["pre_120", "pre_90", "pre_60", "post_60", "post_90", "post_120"]} ], "layer": [ { "mark": "line", "encoding": { "x": { "field": "key", "sort": [ "pre_120", "pre_90", "pre_60", "0", "post_60", "post_90", "post_120" ], "scale": { "domain": [ "pre_120", "pre_90", "pre_60", "0", "post_60", "post_90", "post_120" ] } }, "y": {"field": "value", "type": "quantitative"}, "color": {"field": "type", "type": "nominal"} } }, { "mark": { "type": "rule", "color": "maroon", "size": 3, "strokeDash": [6, 4] }, "encoding": {"x": {"datum": "0"}} } ] }

[1]: https://i.stack.imgur.com/xKCyf.png