It's a "trace" of a variable - usually where several different variables are displayed simultaneously.

El término de búsqueda resulta confuso como “pens”. En mi opinión hay que hablar de “trend pens” o “líneas de datos” que se usan en algunas interfaces HMI (=Human machine interface, “interfaz persona-máquina”), como la del ejemplo de tu pregunta. Como explica el primer enlace de abajo, las interfaces HMI permiten la comunicación entre varios procesadores y recaban la información, los datos, de diversas fuentes. Y hay una serie de conceptos básicos, como “tags” (series de datos), o “trends” (objeto dinámico de forma rectangular que identifica series de datos), o los “trend pens” de tu pregunta, las líneas de datos en forma de objetos dinámicos que el sistema almacena en su memoria y que emplean los “trends”. Cada una de esas líneas es un “trend pen”.

El tema es tan, tan complejo y tan amplio que no voy (ni me atrevo) a profundizar en él. En cualquier caso, yo interpreto “pens” en este contexto como “trend pens”: líneas dinámicas de datos.




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Note added at 1 day9 hrs (2013-03-02 00:01:26 GMT)
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Este documento es excelente para entender el concepto. Contiene un glosario muy útil:
http://www.opto22.com/documents/0626_Mistic_MMI_Manual.pdf
Objects
Most things visible in the Mistic MMI are considered objects, as well as some things not visible. Objects
Most things visible in the Mistic MMI are considered objects, as well as some things not visible. Objects include draw windows, graphics, triggers, and trends.
There are two types of objects: static and dynamic. Static objects have a static appearance and do not change while the MMI Runtime is running. They have no dynamic (changing) attributes. Dynamic objects have a changing appearance or cause the appearance of other MMI objects to change during execution of the MMI Runtime.
Dynamic objects have dynamic attributes defined.
Tags
A Tag is the symbolic name used to identify a piece of data such as an I/O unit, alarm, variable, or system.
Trend
A Trend is a rectangular shaped dynamic object which graphs the change in a variable or set of variables over time. Trends show variables in the vertical axis and time in the horizontal axis.
Trend pen
A trend pen is a dynamic object used by trends. The value of its tag provides the data for the line graphs in the trends.



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Note added at 1 day9 hrs (2013-03-02 00:10:16 GMT)
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http://www.opto22.com.tw/dnl_docs/1702_PAC_Display_Users_Gui...
Working With Basic Trends
You can use basic trends to graph real-time tag information using up to four pens
(B) Pen Configuration Buttons. Use these pen configuration buttons to associate each pen with one PAC Control tag, to define its pen color and width, and to specify a range of values for the tag. You can configure up to four pens per trend. See “Optimizing Pen Settings” on page 151 for more information.

http://www.monitouchhmi.net/pdfs/SVGA_DEMO_rev5.pdf
Graphing – Trend Graph
The Trend Graph will display up to 800 consecutive data points in a line. You can have up to 16 pens on a single graph. Because the data is not buffered, you will specify a starting address and a number of data points. Then the trend will plot the data in a line. This is very useful for data that needs to be displayed for an event that occurs quickly. If you want to display data in 10 millisecond intervals, you can log the data in PLC addresses and then display it on the Trend Graph. Because of the limitations of the cycle time, you would not be able to do that from the buffering area.

la palabra para definirlo puede variar según el país hispano parlante, pero se refiere a esas plumas que registraban trazos en un papel circular o de banda continua en equipos como los de electrocardiogramas, registradores graficos de voltaje u otras variables afines.