Turns out there was a bug...

text-graphics.lisp

;; Alternative graphics: "⣀⣄⣤⣦⣶⣷⣿" or "⠆⡇⡷⣿"
(defun bar (stream width n max &optional (graphics "▏▎▍▌▋▊▉█"))
  "Draw an N/MAX bar no wider than WIDTH using charecters from GRAPHICS."
  (declare (type number width n max)
           (type string graphics))
  (assert (<= 1 (length graphics))
          (graphics)
          "Invalid graphics provided: ~S" graphics)
  (let* ((nchars (length graphics))
         (inc (/ 1 nchars 2)))
    (multiple-value-bind (full part)
        (truncate (* width (/ n max)))
      (loop with char = (char graphics (1- nchars))
            repeat full
            do (write-char char stream))
      (multiple-value-bind (index rem)
          (truncate (* nchars (- part inc)))
        (when (<= 0 rem)
          (write-char (char graphics index) stream)))))
  nil)