front: fix space time chart when first or last waypoint are hidden #14095

Job	Run time
check_generated_railjson_sync	22s
build (core-build, core)	25s
build (editoast, editoast-test)	26s
check_commits	10s
check_railjson_generator	1m 52s
build (gateway-test, gateway-standalone, gateway-front)	1m 18s
build (front-build, front-tests, front-devel, front-nginx)	2m 35s
build (osrdyne, osrdyne-test)	25s
check_integration_tests	1m 24s
Check final newline	7s
check_infra_schema_sync	19s
check_osrd_schema	41s
check_reuse_compliance	10s
check_toml	5s
check_editoast_lints	9m 14s
check_core	4m 3s
check_editoast_tests	6m 39s
check_editoast_openapi	57s
check_front	2m 13s
check_front_rtk_sync	1m 32s
check_gateway	3m 29s
check_osrdyne	3m 45s
integration_tests	11m 52s
	54m 3s

Provide feedback