---
title: "Horizontal Bar Graph"
id: 57250
type: "computer_media"
slug: "horizontal-bar-graph"
url: "http://localhost/computer_media/horizontal-bar-graph/"
markdown_url: "http://localhost/computer_media/horizontal-bar-graph.md"
published_at: "2024-10-04T07:18:06+00:00"
modified_at: "2026-03-30T21:19:00+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/140_HBG.png"
excerpt: "Plot year-by-year data as a horizontal bar chart, with this simple INPUT-driven graphing routine that uses PLOT loops to draw each bar."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
genre:
  - name: "Business"
    slug: "business"
    taxonomy: "genre"
    url: "http://localhost/type/business/"
media_contents:
  - id: 56734
    title: "Timex Sinclair Public Domain Library Tape 1003"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1003/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/140_HBG.png"
media_type_tags: "Business"
---

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.

***

## 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.

## Source Code

```
  10 REM HORIZONTAL BAR GRAPH
  20 PRINT "ENTER STARTING YEAR"
  30 INPUT D
  40 PRINT 
  50 PRINT "ENTER NR BARS DESIRED"
  60 INPUT N
  70 PRINT 
  80 PRINT "ENTER TIME BETWEEN YEARS"
  90 INPUT A
 110 SCROLL 
 120 PRINT TAB 5;"HORIZONTAL BAR GRAPH"
 130 FOR I=1 TO N
 140 SCROLL 
 150 INPUT Z
 160 PRINT D
 170 FOR X=8 TO Z+8
 180 PLOT X,0
 190 NEXT X
 200 PRINT Z
 210 LET D=D+A
 220 NEXT I
 230 STOP 
 240 SAVE "1014%0"
 250 RUN
```