Bar Chart

This program draws a dot-density bar chart on screen using PLOT and FOR/NEXT loops, reading bar data from DATA statements in pairs: bar height and dot spacing. Each bar is rendered as a grid of plotted pixels, with the dot density controlling the spacing between each plotted point in both the horizontal and vertical axes. A border is drawn first via a subroutine at line 400 using DRAW commands. The technique originates from Nick Hampshire’s Color Graphics book.

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Program Structure

The program is divided into four logical sections:

1. Lines 1–4: Introduction text and keypress wait before starting.
2. Lines 60–90: Colour setup (INK, PAPER, BORDER) and subroutine call to draw the border.
3. Lines 100–300: Main bar-chart drawing loop reading DATA pairs and plotting bars.
4. Lines 400–460: Border-drawing subroutine using PLOT/DRAW.

Data Format

Bar data is stored in lines 130–155 as pairs h,d where h is the bar height and d is the dot density (step size). A sentinel pair 0,0 at line 170 terminates the read loop. The six groups of three bars each (18 bars total) are laid out across the screen.

Line	Bar 1 (h,d)	Bar 2 (h,d)	Bar 3 (h,d)
130	20, 1	45, 2	80, 3
135	22, 1	50, 2	70, 3
140	25, 1	35, 2	90, 3
145	35, 1	50, 2	100, 3
150	30, 1	45, 2	75, 3
155	40, 1	55, 2	140, 3

Bar Drawing Technique

Each bar occupies a horizontal span of 6 pixels (b to b+5), and the variable b is incremented by 6 at line 290 after each bar is drawn. The dot density value d is used as the STEP in both the outer x loop (line 240) and the inner y loop (line 250), so a density of 1 produces a solid-looking bar, 2 a medium-density dotted fill, and 3 a sparse dotted fill. This elegantly encodes fill style directly into the plotting loops without requiring any additional conditional logic.

Border Subroutine

The subroutine at lines 400–460 draws a rectangular border around the full 256×176 pixel plotting area using a sequence of DRAW commands. Note that the screen plotting area is 255×175 pixels (0–255 wide, 0–175 tall), so the DRAW 255,0 and DRAW 0,175 calls reach the far edges correctly from the origin at PLOT 0,0.

Key BASIC Idioms

• Line 3 uses IF INKEY$="" THEN GO TO 3 as a simple keypress wait — a standard Sinclair idiom.
• The sentinel DATA 0,0 at line 170 with the check IF d=0 THEN STOP at line 230 is a clean loop termination pattern for DATA-driven programs.
• GO TO 220 at line 300 re-enters the READ loop without a FOR/NEXT counter, relying on the DATA pointer advancing automatically.