This affects only those premade charts that have not been set to a custom position via chart.Position_worldspace(todo link). If you want a chart for which you have set the position manually to get back to automatic positioning and layout you can use RevertPremadeChartsToAutomaticPositionLayout(). After that all lines will be affected again by this "SetWidth_forAllPremadeLineCharts" function.
If the "newWidth" parameter is left at its default value, then the width will get reverted to the default width as it was before doing any changes.