如何提取GNUPLOT中set label命令文本末尾的屏幕坐标？

Great question—this is a super common pain point when working with dynamic text in gnuplot, especially since font type, size, terminal, and scaling all make manual position calculations impossible. Luckily, gnuplot has built-in tools to handle this dynamically.

Here's how to reliably get the end screen coordinates of your label, step by step:

Core Tools: strwidth() and Gnuplot's Internal Variables #

Gnuplot’s strwidth(string) function returns the width of a string in your plot’s data units, and auto-populated GPVAL_* variables track your plot’s screen and data ranges. Combining these lets you convert text width to screen coordinates without manual math.

Example Workflow #

Let’s walk through placing a label, finding its end position, and adding an arrow right after it:

# 1. Define your text and starting screen position
my_label_text = "Gnuplot is awesome!"
start_x_screen = 0.2  # 20% from the terminal's left edge
start_y_screen = 0.7  # 70% from the terminal's bottom edge

# 2. Set your label using screen coordinates for consistency
set label 1 my_label_text at screen start_x_screen, start_y_screen

# 3. Convert text width from data units to screen units
text_width_data = strwidth(my_label_text)
data_range_x = GPVAL_X_MAX - GPVAL_X_MIN
screen_range_x = GPVAL_X_SCREEN_MAX - GPVAL_X_SCREEN_MIN
text_width_screen = text_width_data * (screen_range_x / data_range_x)

# 4. Calculate the label's end screen coordinates
end_x_screen = start_x_screen + text_width_screen
end_y_screen = start_y_screen  # stays the same for single-line text

# 5. Add your desired element at the end (e.g., a red arrow)
set arrow 1 from screen end_x_screen, end_y_screen to screen end_x_screen + 0.1, end_y_screen lc "red" lw 2

# Generate a test plot to see the result
plot x title "Sample Plot"

Handling Data Coordinates #

If you placed your label using data coordinates instead of screen coordinates, first convert the start position to screen coordinates with the screen() function:

start_x_data = 5
start_y_data = 10
start_x_screen = screen(start_x_data)
start_y_screen = screen(start_y_data)

Then follow the same width calculation steps above.

Edge Case Notes #

• Multi-line text: Use strheight(string) to calculate the height of multi-line text if you need bottom/top end coordinates.
• Terminal/font auto-adjustment: strwidth() automatically accounts for your terminal type, font, and font size—no manual adjustments needed for these variables.
• Version compatibility: This method works in Gnuplot 5.0 and newer, which is standard for most users today.

内容的提问来源于stack exchange，提问作者Bo Sundman