Horizontal Bar Graph

This program renders a horizontal bar graph on screen, prompting the user to enter a starting year, the number of bars, and a time increment between years. For each bar, it accepts a numeric value and draws a horizontal line of plotted points using the ZX81’s PLOT command, starting at column 8 and extending to column Z+8. The year label is printed to the left of each bar, and the numeric value is printed to the right, with the display scrolling between entries. The use of PLOT within a FOR loop to build each bar is a characteristic technique for producing rudimentary graphics on text-based display hardware.

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Program Analysis

Program Structure

The program is a straightforward linear sequence with two nested FOR loops. The outer loop (lines 130–220) iterates N times, once per bar. The inner loop (lines 170–190) draws each individual bar using PLOT. After all bars are drawn, the program halts at line 230.

1. Initialisation (lines 20–90): Collects starting year (D), number of bars (N), and year increment (A) via INPUT.
2. Header (lines 110–120): Scrolls and prints the graph title centred with TAB 5.
3. Bar drawing loop (lines 130–220): For each iteration, scrolls the display, inputs value Z, prints the current year label D, plots the bar, prints Z as a right-side label, and advances the year by A.

Variable Usage

Variable	Purpose
D	Current year label, incremented by A each iteration
N	Total number of bars to draw
A	Year increment between bars
I	Outer loop counter
Z	Data value for current bar
X	Inner loop counter / horizontal plot position

PLOT-Based Bar Drawing

Each bar is drawn by the inner FOR X=8 TO Z+8 loop, calling PLOT X,0 repeatedly. The bar always starts at column 8 (leaving room for the year label printed just before) and extends to column Z+8, so the bar length in pixels equals Z. The Y coordinate is fixed at 0, meaning all points are plotted on a single pixel row. This produces a thin one-pixel-high horizontal line rather than a filled bar.

Key BASIC Idioms

• SCROLL before each bar: SCROLL at lines 110 and 140 shifts the display up one line, preventing the screen from filling and pausing — an important consideration on hardware with no automatic scroll prompt control.
• Interleaved labels and graphics: Printing D (year) before the PLOT loop and Z (value) after provides left and right annotations for each bar, though both appear on separate text lines rather than inline with the plotted pixels.
• Year auto-increment: LET D=D+A at line 210 advances the year counter generically, supporting any regular interval (annual, biennial, decadal, etc.).

Bugs and Anomalies

• No input validation: If Z is negative, the FOR X=8 TO Z+8 loop body never executes and no bar is drawn, silently producing an empty row.
• Fixed Y coordinate: PLOT X,0 always plots at row 0. On a ZX81, row 0 is at the bottom of the graphics area, so all bars share a common baseline, which is actually correct for a bar chart — however, the bars are only one pixel tall, making them visually thin lines rather than solid bars.
• Screen width limit: The ZX81 display is 64 pixels wide. If Z exceeds approximately 55, the plot coordinates will overflow the screen without error checking, potentially causing an out-of-screen-bounds error or wrapping, depending on the implementation.
• Line 100 is missing: The line numbering jumps from 90 to 110, leaving a gap that has no functional effect but suggests a line was deleted during editing.