If you've ever used Tableau, you've probably tried to drag a date onto the shelf expecting line graph but got bars instead, or vice versa. Either way, you're left wondering what you did wrong.
Most of the time, it's due to whether the date field is blue or green. Tableau calls these discrete and continuous dates. The difference between them affects pretty much everything about how a date behaves on a chart. This blog covers what each colour means, why Tableau treats them differently, and how to pick the right one depending on what you're trying to show.
How Tableau Treats Dates By Default
When you connect to a database in Tableau Desktop, it automatically detects date or datetime fields, assigns the correct data type, and pops a calendar icon next to the field name. From there Tableau lets you work with dates in two different ways:
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Date parts: A specific date component like Month or Weekday, with the rest of the date removed. If you're working with the month part, every January gets treated as the same "January" regardless of year
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Date values (truncated dates): A specific point in time, like "January 2024", which is distinct from "January 2025". This preserves the data's timeline, so nothing gets merged.
Take March as an example. As a date part, March 2025 and March 2026 collapse into one bucket called "March". As a date value, they're plotted as separate points along the timeline. These fields contain identical date information but produce different results depending on how you've set them up.
Discrete Dates (Blue Fields)
When a date field is discrete, Tableau treats it as a category (like a name, label, or a group). Discrete fields are always blue. When placed on a shelf, they create separate headers for each value rather than a continuous axis.
In practice this looks like:
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Dragging a discrete Month field onto columns produces individual bars or headers for January, February, March etc.
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Dragging a discrete Year field alongside it lets you compare months across multiple years, side by side.
When to use: Use discrete dates to compare distinct periods, rather than finding trends (e.g., comparing March across three years).
Continuous Dates (Green Fields)
When a field is continuous, Tableau treats it as an unbroken range of dates. Continuous fields are always green, and when placed on a shelf they create an axis rather than headers.
In practice this looks like:
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Line charts with continuous axes that flow from one point to the next, showing trends and patterns over time.
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Missing data that can create gaps in the line graph (e.g. if there was one month without sales).
When to use: Use continuous dates for trends, like website traffic, revenue over time, or forecasting, where the shape of the line matters more than comparing discrete bins.
Choosing Which One To Use
The decision usually comes down to one question:
"Am I comparing categories or looking for trends?"
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Comparing → choose discrete (blue)."How does this month stack up compared to the same month last year?
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Finding trends → choose continuous (green). "How has this metric changed over time?"
Tableau makes it easy to switch between the two. Right-click any date field on a shelf and click discrete or continuous dates from the menu, and the visualisation updates immediately. This is worth playing with as the same data can tell two very different stories depending on the field type.
If your chart doesn't look right, check the colour of the date field first. Once you're comfortable with what blue and green fields mean, the way dates behave in Tableau starts to make a lot more sense.