What are the most common mistakes when creating line graphs?

Almost no line charts use the same units on both axes. This should probably have read something like, "All values in the chart should have the same unit on the x-axis and the same unit on the y-axis."

Instead of using a legend, try to label lines directly, whenever possible. This avoids forcing the audience to memorize colors.

Thanks @Jessica Enders. Good points. A few responses: 1. If you frame the chart area with a border, that border only does 1 thing. Gridlines frame the chart area and do several other things without adding much more clutter (many charts only have four or five gridlines, i.e., the same number of lines as a rectangular border). 2. If the main message is vague like "This metric has been increasing over time," then, yes, gridlines wouldn't be necessary, but, in my experience, many audiences think ALL charts have gridlines and might wonder why they're not there. 3. Most line charts are "horizontal," but other chart types (i.e. bars) can be either. In a dashboard/report, then, it might not be clear why some charts have gridlines and others don't.

@ Nick Desbarats With regard to your points: 1. What do you think about framing the chart area to accomplish the same thing? Would be more minimal than gridlines. 2. Maybe it depends on the purpose of the graph? If only a broad direction is needed and/or users are going to refer to supplementary tables when they need exact values, then gridlines may not be necessary? 3. In my experience, most line graphs are read horizontally and users intuitively expect higher values the be higher up and lower values to be lower down. Vertical line graphs are far less common. So perhaps gridlines are less necessary in horizontal line graphs but essential in vertical ones?

I don't recommend removing gridlines since they serve three important purposes: 1. They "frame" the chart area, i.e., the area where lines could be in the chart. 2. They allow values on the lines to be estimated more easily and precisely, especially values that are further from the quantitative scale. 3. They immediately indicate the "quantitative direction" of the chart: Horizontal gridlines indicate that values that are higher up in the chart represent greater quantities, while vertical gridlines indicate that values that are further to the right represent greater quantities (e.g., in a bar chart with horizontal bars).

Menos es mas, una premisa para todo tipo de gráfico, evitar sobrecargar la visual con múltiples líneas, mas allá de las 5, de preferencia se crean filtros/tablas/gráficos auxiliares para mostrar las informaciones adicionales o complementarias, importante el uso de colores relacionados con la información (por ejemplo fechas pasadas en colores claros, cercanas al presente en oscuros, evitar los arcoíris), también el uso pertinente de colores conocidos por su significado de acuerdo al área de trabajo, ejemplo el rojo en muchos casos es información negativa al contrario del negro, pero en administración de emergencias y desastres el color negro representa muertes y rojo es un valor prioritario.