Starting as a finger-picking folkie and winding up as a jazz-savvy experimentalist, Joni Mitchell has brought a sharp eye, light touch and an agile trill of a voice to her songs, which have often dissected her romances and skewered myopic government. A wealth of musicians — from Tori Amos to Prince, from Joanna Newsom to Iron & Wine and even Michael Hedges — have acknowledged her influence.