Very cool! But what does "accessibility" mean? The tool tip isn't helpful. What's the scale? Is it linear, exponential, or logarithmic? What are the end points

Very cool! But what does "accessibility" mean? The tool tip isn't helpful. What's the scale? Is it linear, exponential, or logarithmic? What are the end points? I.e., does a bigger number here mean it's more accessible or less accessible than a smaller number? Also, could you please explain how you're calculating the cost of extraction and transportation? Thank you for putting this together!